fix($httpBackend): allow canceling request with falsy timeoutId

httpBackend with ngMock browser.defer could 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.

Closes #10177

Author: gyx1000

src/ng/httpBackend.js

@@ -126,7 +126,9 @@ 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);
+      if (timeoutId !== undefined) {
+        $browserDefer.cancel(timeoutId);
+      }
       jsonpDone = xhr = null;
 
       callback(status, response, headersString, statusText);