test(XHRConnection): normalize responseText and response

normalize xhr.responseText and xhr.response

- [x] Tests

Closes #2882

modules/angular2/src/render/xhr_impl.ts

@@ -1,5 +1,6 @@
 import {Injectable} from 'angular2/di';
 import {Promise, PromiseWrapper, PromiseCompleter} from 'angular2/src/facade/async';
+import {isPresent} from 'angular2/src/facade/lang';
 import {XHR} from './xhr';
 
 @Injectable()
@@ -13,7 +14,7 @@ export class XHRImpl extends XHR {
     xhr.onload = function() {
       // responseText is the old-school way of retrieving response (supported by IE8 & 9)
       // response/responseType properties were introduced in XHR Level2 spec (supported by IE10)
-      var response = ('response' in xhr) ? xhr.response : xhr.responseText;
+      var response = isPresent(xhr.response) ? xhr.response : xhr.responseText;
 
       // normalize IE9 bug (http://bugs.jquery.com/ticket/1450)
       var status = xhr.status === 1223 ? 204 : xhr.status;