fix: allow setting anonymousId

Historically, the iOS library has allowed setting a custom anonymous identifier (#485).

During adding middlewares, a bug was introduced which started ignoring the anonymousId option.

This fixes the bug and adds a test to validate the fix. This was also tested in the example app end to end and I've verified that events show up with the custom anonymousId.

Test snipet:

```
[[SEGAnalytics sharedAnalytics] identify:@"Prateek" traits:nil options: @{
                                                                           @"anonymousId":@"test_anonymousId"
                                                                         }];
```

Event in debugger: https://cloudup.com/cFyVH4NPR_f

Subsequent events will automatically attach the new anonymous ID https://cloudup.com/cK-i_SCbwsN

Analytics/Classes/Integrations/SEGIntegrationsManager.m

@@ -541,7 +541,15 @@ - (void)context:(SEGContext *)context next:(void (^_Nonnull)(SEGContext *_Nullab
     switch (context.eventType) {
         case SEGEventTypeIdentify: {
             SEGIdentifyPayload *p = (SEGIdentifyPayload *)context.payload;
-            [self identify:p.userId traits:p.traits options:p.options];
+            NSDictionary *options;
+            if (p.anonymousId) {
+                NSMutableDictionary *mutableOptions = [[NSMutableDictionary alloc] initWithDictionary:p.options];
+                mutableOptions[@"anonymousId"] = p.anonymousId;
+                options = [mutableOptions copy];
+            } else {
+                options =  p.options;
+            }
+            [self identify:p.userId traits:p.traits options:options];
             break;
         }
         case SEGEventTypeTrack: {

Analytics/Classes/SEGAnalytics.m

@@ -201,7 +201,7 @@ - (void)identify:(NSString *)userId traits:(NSDictionary *)traits options:(NSDic
     NSCAssert2(userId.length > 0 || traits.count > 0, @"either userId (%@) or traits (%@) must be provided.", userId, traits);
     [self run:SEGEventTypeIdentify payload:
                                        [[SEGIdentifyPayload alloc] initWithUserId:userId
-                                                                      anonymousId:nil
+                                                                      anonymousId:[options objectForKey:@"anonymousId"]
                                                                            traits:SEGCoerceDictionary(traits)
                                                                           context:SEGCoerceDictionary([options objectForKey:@"context"])
                                                                      integrations:[options objectForKey:@"integrations"]]];

AnalyticsTests/TrackingTests.swift

@@ -15,7 +15,7 @@ class TrackingTests: QuickSpec {
   override func spec() {
     var passthrough: SEGPassthroughMiddleware!
     var analytics: SEGAnalytics!
-    
+
     beforeEach {
       let config = SEGAnalyticsConfiguration(writeKey: "QUI5ydwIGeFFTa1IvCBUhxL9PyW5B0jE")
       passthrough = SEGPassthroughMiddleware()
@@ -24,21 +24,35 @@ class TrackingTests: QuickSpec {
       ]
       analytics = SEGAnalytics(configuration: config)
     }
-    
+
     afterEach {
       analytics.reset()
     }
-    
+
     it("handles identify:") {
       analytics.identify("testUserId1", traits: [
         "firstName": "Peter"
       ])
       expect(passthrough.lastContext?.eventType) == SEGEventType.identify
       let identify = passthrough.lastContext?.payload as? SEGIdentifyPayload
       expect(identify?.userId) == "testUserId1"
+      expect(identify?.anonymousId).to(beNil())
+      expect(identify?.traits?["firstName"] as? String) == "Peter"
+    }
+
+    it("handles identify with custom anonymousId:") {
+      analytics.identify("testUserId1", traits: [
+        "firstName": "Peter"
+        ], options: [
+          "anonymousId": "a_custom_anonymous_id"
+        ])
+      expect(passthrough.lastContext?.eventType) == SEGEventType.identify
+      let identify = passthrough.lastContext?.payload as? SEGIdentifyPayload
+      expect(identify?.userId) == "testUserId1"
+      expect(identify?.anonymousId) == "a_custom_anonymous_id"
       expect(identify?.traits?["firstName"] as? String) == "Peter"
     }
-    
+
     it("handles track:") {
       analytics.track("User Signup", properties: [
         "method": "SSO"
@@ -48,14 +62,14 @@ class TrackingTests: QuickSpec {
       expect(payload?.event) == "User Signup"
       expect(payload?.properties?["method"] as? String) == "SSO"
     }
-    
+
     it("handles alias:") {
       analytics.alias("persistentUserId")
       expect(passthrough.lastContext?.eventType) == SEGEventType.alias
       let payload = passthrough.lastContext?.payload as? SEGAliasPayload
       expect(payload?.theNewId) == "persistentUserId"
     }
-    
+
     it("handles screen:") {
       analytics.screen("Home", properties: [
         "referrer": "Google"
@@ -65,7 +79,7 @@ class TrackingTests: QuickSpec {
       expect(screen?.name) == "Home"
       expect(screen?.properties?["referrer"] as? String) == "Google"
     }
-    
+
     it("handles group:") {
       analytics.group("acme-company", traits: [
         "employees": 2333

Examples/CocoapodsExample/CocoapodsExample/AppDelegate.m

@@ -29,7 +29,11 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
     configuration.trackAttributionData = YES;
     configuration.flushAt = 1;
     [SEGAnalytics setupWithConfiguration:configuration];
+    [[SEGAnalytics sharedAnalytics] identify:@"Prateek" traits:nil options: @{
+                                                                              @"anonymousId":@"test_anonymousId"
+                                                                              }];
     [[SEGAnalytics sharedAnalytics] track:@"Cocoapods Example Launched"];
+
     [[SEGAnalytics sharedAnalytics] flush];
     NSLog(@"application:didFinishLaunchingWithOptions: %@", launchOptions);
     return YES;