Merge pull request jashkenas#2515 from CodeFalling/safer-tostring

safer _#toString, fix jashkenas#2498

underscore.js

@@ -1608,7 +1608,7 @@
   _.prototype.valueOf = _.prototype.toJSON = _.prototype.value;
 
   _.prototype.toString = function() {
-    return '' + this._wrapped;
+    return String(this._wrapped);
   };
 
   // AMD registration happens at the end for compatibility with AMD loaders