fix(http): Set response.ok

The ok property was not being set on response objects.
It's now set based on the status code.

Closes #6390
Closes #6503

modules/@angular/http/src/backends/xhr_backend.ts

@@ -69,7 +69,8 @@ export class XHRConnection implements Connection {
           responseOptions = baseResponseOptions.merge(responseOptions);
         }
         let response = new Response(responseOptions);
-        if (isSuccess(status)) {
+        response.ok = isSuccess(status);
+        if (response.ok) {
           responseObserver.next(response);
           // TODO(gdi2290): defer complete if array buffer until done
           responseObserver.complete();