add property 'protocol' to make it work under node 0.11

Raise `Protocol http: not supported` error under node 0.11.

See details in [joyent/node/_http_client.js#L63](https://github.com/joyent/node/blob/master/lib%2F_http_client.js#L63) and [joyent/node/_http_client.js#L73](https://github.com/joyent/node/blob/master/lib%2F_http_client.js#L73)
