Makes HTTPResponse serializable (#2143)

* Use the callback body instead of response.body that may not be set

* Adds test to handle undefined responses

* Adds toJSON method to properly serialize HTTPResponse

* Use ES5 defineProperty to make keys enumerable

* removes body key from serialization

* Indent nits

src/cloud-code/httpRequest.js

@@ -62,7 +62,7 @@ module.exports = function(options) {
       }
       return promise.reject(error);
     }
-    let httpResponse = new HTTPResponse(response);
+    let httpResponse = new HTTPResponse(response, body);
 
     // Consider <200 && >= 400 as errors
     if (httpResponse.status < 200 || httpResponse.status >= 400) {