fix($httpBackend): Set current url, if not defined or empty string

Reason to fix this was the fact that with undefined url, it ended up with weird exception
(Cannot call method 'replace' of undefined), which was more confusing than helpful.

jQuery.ajax() does request to current url, if url is not specified, so I decided for this solution.

src/service/httpBackend.js

@@ -34,6 +34,7 @@ function createHttpBackend($browser, XHR, $browserDefer, callbacks, body, locati
   // TODO(vojta): fix the signature
   return function(method, url, post, callback, headers, timeout) {
     $browser.$$incOutstandingRequestCount();
+    url = url || $browser.url();
 
     if (lowercase(method) == 'jsonp') {
       var callbackId = '_' + (callbacks.counter++).toString(36);

test/service/httpBackendSpec.js

@@ -168,6 +168,17 @@ describe('$httpBackend', function() {
     });
 
 
+    it('should set url to current location if not specified or empty string', function() {
+      $backend('JSONP', undefined, null, callback);
+      expect($browser.$$scripts[0].url).toBe($browser.url());
+      $browser.$$scripts.shift();
+
+      $backend('JSONP', '', null, callback);
+      expect($browser.$$scripts[0].url).toBe($browser.url());
+      $browser.$$scripts.shift();
+    });
+
+
     // TODO(vojta): test whether it fires "async-start"
     // TODO(vojta): test whether it fires "async-end" on both success and error
   });