Optimize isKey.

lodash.src.js

@@ -89,6 +89,7 @@
 
   /** Used to match property names within property paths. */
   var reIsDeepProp = /\.|\[(?:[^[\]]+|(["'])(?:(?!\1)[^\n\\]|\\.)*?)\1\]/,
+      reIsPlainProp = /^\w*$/,
       rePropName = /[^.[\]]+|\[(?:(-?\d+(?:\.\d+)?)|(["'])((?:(?!\2)[^\n\\]|\\.)*?)\2)\]/g;
 
   /**
@@ -4330,6 +4331,10 @@
      * @returns {boolean} Returns `true` if `value` is a property name, else `false`.
      */
     function isKey(value, object) {
+      var type = typeof value;
+      if ((type == 'string' && reIsPlainProp.test(value)) || type == 'number') {
+        return true;
+      }
       if (isArray(value)) {
         return false;
       }