merging #532 -- allow string as iterator on sortBy

test/collections.js

@@ -238,6 +238,10 @@ $(document).ready(function() {
 
     var list = [undefined, 4, 1, undefined, 3, 2];
     equal(_.sortBy(list, _.identity).join(','), '1,2,3,4,,', 'sortBy with undefined values');
+
+    var list = ["one", "two", "three", "four", "five"];
+    var sorted = _.sortBy(list, 'length');
+    equal(sorted.join(' '), 'one two four five three', 'sorted by length');
   });
 
   test('collections: groupBy', function() {

underscore.js

@@ -262,7 +262,8 @@
   };
 
   // Sort the object's values by a criterion produced by an iterator.
-  _.sortBy = function(obj, iterator, context) {
+  _.sortBy = function(obj, val, context) {
+    var iterator = _.isFunction(val) ? val : function(obj) { return obj[val]; };
     return _.pluck(_.map(obj, function(value, index, list) {
       return {
         value : value,