Finish the previous _.bind revert.

test/functions.js

@@ -37,6 +37,7 @@ $(document).ready(function() {
     var newBoundf = new Boundf();
     equal(newBoundf.hello, undefined, "function should not be bound to the context, to comply with ECMAScript 5");
     equal(Boundf().hello, "moe curly", "When called without the new operator, it's OK to be bound to the context");
+    ok(newBoundf instanceof F, "a bound instance is an instance of the original function");
   });
 
   test("bindAll", function() {

underscore.js

@@ -573,16 +573,19 @@
   // optionally). Binding with arguments is also known as `curry`.
   // Delegates to **ECMAScript 5**'s native `Function.bind` if available.
   // We check for `func.bind` first, to fail fast when `func` is undefined.
-  _.bind = function bind(func, context) {
+  _.bind = function(func, context) {
     var args, bound;
     if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
     if (!_.isFunction(func)) throw new TypeError;
     args = slice.call(arguments, 2);
     return bound = function() {
       if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
-      var result = func.apply(this, args.concat(slice.call(arguments)));
+      ctor.prototype = func.prototype;
+      var self = new ctor;
+      ctor.prototype = null;
+      var result = func.apply(self, args.concat(slice.call(arguments)));
       if (Object(result) === result) return result;
-      return this;
+      return self;
     };
   };