Make _.defaults avoid accessing property values it doesn't need to. [closes #2983]

lodash.js

@@ -12774,9 +12774,35 @@
      * _.defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
      * // => { 'a': 1, 'b': 2 }
      */
-    var defaults = baseRest(function(args) {
-      args.push(undefined, customDefaultsAssignIn);
-      return apply(assignInWith, undefined, args);
+    var defaults = baseRest(function(object, sources) {
+      object = Object(object);
+
+      var index = -1;
+      var length = sources.length;
+      var guard = length > 2 ? sources[2] : undefined;
+
+      if (guard && isIterateeCall(sources[0], sources[1], guard)) {
+        length = 1;
+      }
+
+      while (++index < length) {
+        var source = sources[index];
+        var props = keysIn(source);
+        var propsIndex = -1;
+        var propsLength = props.length;
+
+        while (++propsIndex < propsLength) {
+          var key = props[propsIndex];
+          var value = object[key];
+
+          if (value === undefined ||
+              (eq(value, objectProto[key]) && !hasOwnProperty.call(object, key))) {
+            object[key] = source[key];
+          }
+        }
+      }
+
+      return object;
     });
 
     /**