fix typoe

lib/promise.js

@@ -11,7 +11,7 @@ Promise.prototype._then = function (didFulfill, didReject, didProgress, receiver
     if (typeof didProgress === 'function') didProgress = ns.bind(didProgress);
   }
   
-  return then.call(this, didFulfill, didReject, didProgress, receiver, internalData);
+  return _then.call(this, didFulfill, didReject, didProgress, receiver, internalData);
 };
 
 module.exports = Promise;
\ No newline at end of file