fix($httpBackend): cancelled JSONP requests will not print error in the console

When you cancel a JSONP request, angular deletes the callback for it. However the script still executes, and since the callback is now deleted and undefined, the script throws an exception visible in the console. The quick fix for this is not to delete the callback, but replace it with `angular.noop`.

Closes angular#5615
Closes angular#5616

Author: Artemy Tregubenko

src/ng/httpBackend.js

@@ -54,7 +54,7 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
         } else {
           completeRequest(callback, status || -2);
         }
-        delete callbacks[callbackId];
+        callbacks[callbackId] = angular.noop;
       });
     } else {
 

test/ng/httpBackendSpec.js

@@ -330,7 +330,7 @@ describe('$httpBackend', function() {
         script.onload();
       }
 
-      expect(callbacks[callbackId]).toBeUndefined();
+      expect(callbacks[callbackId]).toBe(angular.noop);
       expect(fakeDocument.body.removeChild).toHaveBeenCalledOnceWith(script);
     });
 
@@ -397,7 +397,7 @@ describe('$httpBackend', function() {
     });
 
 
-    it('should abort request on timeout', function() {
+    it('should abort request on timeout and replace callback with noop', function() {
       callback.andCallFake(function(status, response) {
         expect(status).toBe(-1);
       });
@@ -406,9 +406,14 @@ describe('$httpBackend', function() {
       expect(fakeDocument.$$scripts.length).toBe(1);
       expect(fakeTimeout.delays[0]).toBe(2000);
 
+      var script = fakeDocument.$$scripts.shift(),
+        callbackId = script.src.match(SCRIPT_URL)[2];
+
       fakeTimeout.flush();
       expect(fakeDocument.$$scripts.length).toBe(0);
       expect(callback).toHaveBeenCalledOnce();
+
+      expect(callbacks[callbackId]).toBe(angular.noop);
     });