Ensure _.bindKey funcs call object[key] when called with the new operator.

lodash.js

@@ -4084,7 +4084,7 @@
           args.length = ary;
         }
         if (this && this !== root && this instanceof wrapper) {
-          fn = Ctor || createCtorWrapper(func);
+          fn = Ctor || createCtorWrapper(fn);
         }
         return fn.apply(thisBinding, args);
       }