Alias mapObject to mapValues

For the record, I like `_.mapObject` better. But this change was
undiscussed.

Re: #2061, 4f771e0

index.html

@@ -1370,6 +1370,7 @@ <h2 id="objects">Object Functions</h2>
 
 <p id="mapObject">
         <b class="header">mapObject</b><code>_.mapObject(array, iteratee, [context])</code>
+        <span class="alias">Alias: <b>mapValues</b></span>
         <br />
         Like <a href="#map">map</a>, but for objects. Transform the value
         of each property in turn.

underscore.js

@@ -951,7 +951,7 @@
 
   // Returns the results of applying the iteratee to each element of the object
   // In contrast to _.map it returns an object
-  _.mapObject = function(obj, iteratee, context) {
+  _.mapObject =_.mapValues = function(obj, iteratee, context) {
     iteratee = cb(iteratee, context);
     var keys =  _.keys(obj),
           length = keys.length,