fix($httpBackend): don't delete xhr.onreadystatechange otherwise Safa…

…ri :-O

src/ng/httpBackend.js

@@ -71,14 +71,14 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
       // response is in the cache. the promise api will ensure that to the app code the api is
       // always async
       xhr.onreadystatechange = function() {
-        if (xhr.readyState == 4) {
-          // onreadystatechange might by called multiple times
-          // with readyState === 4 on mobile webkit caused by
-          // xhrs that are resolved while the app is in the background (see #5426).
-          //
-          // we must delete the property instead of setting it to undefined/null to make IE8 happy.
-          delete xhr.onreadystatechange;
-
+        // onreadystatechange might by called multiple times with readyState === 4 on mobile webkit caused by
+        // xhrs that are resolved while the app is in the background (see #5426).
+        // since calling completeRequest sets the `xhr` variable to null, we just check if it's not null before
+        // continuing
+        //
+        // we can't set xhr.onreadystatechange to undefined or delete it because that breaks IE8 (method=PATCH) and
+        // Safari respectively.
+        if (xhr && xhr.readyState == 4) {
           var responseHeaders = null,
               response = null;
 

test/ng/httpBackendSpec.js

@@ -93,14 +93,16 @@ describe('$httpBackend', function() {
   // onreadystatechange might by called multiple times
   // with readyState === 4 on mobile webkit caused by
   // xhrs that are resolved while the app is in the background (see #5426).
-  it('should remove onreadystatechange when it is called with readyState=4 to ignore multiple calls', function() {
+  it('should not process onreadystatechange callback with readyState == 4 more than once', function() {
     $backend('GET', 'URL', null, callback);
     xhr = MockXhr.$$lastInstance;
 
     xhr.status = 200;
     xhr.readyState = 4;
     xhr.onreadystatechange();
-    expect(xhr.onreadystatechange).toBeUndefined();
+    xhr.onreadystatechange();
+
+    expect(callback).toHaveBeenCalledOnce();
   });
 
   it('should set only the requested headers', function() {