Use utils.extend instead of Object.assign

... because javascript is awful

lib/interactive-auth.js

@@ -18,6 +18,8 @@ limitations under the License.
 /** @module interactive-auth */
 var q = require("q");
 
+var utils = require("./utils");
+
 /**
  * Abstracts the logic used to drive the interactive auth process.
  *
@@ -116,7 +118,7 @@ InteractiveAuth.prototype = {
         var auth = {
             session: this._data.session,
         };
-        Object.assign(auth, authData);
+        utils.extend(auth, authData);
 
         this._doRequest(auth);
     },