Fix(http): invalidStateError response body

Check on null value failed with last version of mozilla.
Check on undefined type instead.

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

@@ -59,7 +59,7 @@ export class XHRConnection implements Connection {
           // responseText is the old-school way of retrieving response (supported by IE8 & 9)
           // response/responseType properties were introduced in ResourceLoader Level2 spec
           // (supported by IE10)
-          body = _xhr.response == null ? _xhr.responseText : _xhr.response;
+          body = (typeof _xhr.response === 'undefined') ? _xhr.responseText : _xhr.response;
 
           // Implicitly strip a potential XSSI prefix.
           if (typeof body === 'string') {