Merge pull request #1394 from mcbex/master

assign value to new arrays at creation in _.groupBy

underscore.js

@@ -356,7 +356,7 @@
   // Groups the object's values by a criterion. Pass either a string attribute
   // to group by, or a function that returns the criterion.
   _.groupBy = group(function(result, key, value) {
-    (_.has(result, key) ? result[key] : (result[key] = [])).push(value);
+    _.has(result, key) ? result[key].push(value) : result[key] = [value];
   });
 
   // Indexes the object's values by a criterion, similar to `groupBy`, but for