Support ref.push() without arguments

The example in the docs shows this use: https://firebase.google.com/docs/reference/js/firebase.database.Reference#push

Calling .push() should create a child and return a reference to it. Currently you're returning null, so it looks like a pretty small change to support this behaviour.

Author: adrianosmond

src/firebase.js

@@ -240,7 +240,7 @@ MockFirebase.prototype.push = function (data, callback) {
     validate.data(data);
     return utils.createThenableReference(child, child.set(data, callback));
   } else {
-    return utils.createThenableReference(child, Promise.resolve(null));
+    return utils.createThenableReference(child, Promise.resolve(child));
   }
 };