fix($httpBackend): compare timeoutId with undefined in completeRequest

httpBackend with ngMock browser.defer can never cancel the first deferredFn because the timeoutId returned by defer for the first fn is a zero value.
Compare timeoutId with undefined fix this issue.

Author: gyx1000

src/ng/httpBackend.js

@@ -126,7 +126,7 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
 
     function completeRequest(callback, status, response, headersString, statusText) {
       // cancel timeout and subsequent timeout promise resolution
-      timeoutId && $browserDefer.cancel(timeoutId);
+      timeoutId !== undefined && $browserDefer.cancel(timeoutId);
       jsonpDone = xhr = null;
 
       callback(status, response, headersString, statusText);