update push to return thenable reference

resolve soumak77#83

Author: soumak77

src/firebase.js

@@ -238,9 +238,10 @@ MockFirebase.prototype.push = function (data, callback) {
   if (arguments.length && data !== null) {
     // currently, callback only invoked if child exists
     validate.data(data);
-    child.set(data, callback);
+    return utils.createThenableReference(child, child.set(data, callback));
+  } else {
+    return utils.createThenableReference(child, Promise.resolve(null));
   }
-  return child;
 };
 
 MockFirebase.prototype.once = function (event, callback, cancel, context) {

src/utils.js

@@ -199,3 +199,10 @@ exports.findUndefinedProperties = function (obj) {
   recurse(obj, path);
   return results;
 };
+
+exports.createThenableReference = function(reference, promise) {
+  reference.then = function(success, failure) {
+    return promise.then(success).catch(failure);
+  };
+  return reference;
+};

test/unit/firebase.js

@@ -874,6 +874,19 @@ describe('MockFirebase', function () {
       expect(ref.push.bind(ref, {someProp: undefined})).to.throw();
     });
 
+    it('should return thenable reference', function (done) {
+      var thenable = ref.push({
+        foo: 'bar'
+      });
+      thenable.then(function() {
+        expect(thenable.getData()).to.deep.equal({
+          foo: 'bar'
+        });
+        done();
+      }, done);
+      ref.flush();
+    });
+
     it('can add data by auto id', function () {
       var id = ref._newAutoId();
       sinon.stub(ref, '_newAutoId').returns(id);