small bugfix

It inherits from `stream` : https://github.com/mikeal/request/blob/7430bf38ad032f732cced048570634d8de42cec0/request.js#L102 

I've noticed differences between http.clientRequest and request. request returns the stream whereas clientRequest takes a callback (http://nodejs.org/api/http.html#http_http_request_options_callback) if I am not mistaken.