Issue #149: _.keys should throw a TypeError for non-objects.

underscore.js

@@ -504,6 +504,7 @@
   // Retrieve the names of an object's properties.
   // Delegates to **ECMAScript 5**'s native `Object.keys`
   _.keys = nativeKeys || function(obj) {
+    if (obj !== Object(obj)) throw new TypeError('Invalid object');
     var keys = [];
     for (var key in obj) if (hasOwnProperty.call(obj, key)) keys[keys.length] = key;
     return keys;