Enable websocket cross-origin while ws_origin is set as ""

[yejianquan]

Hi Dai,

Gotty is an awesome project and I like it very much, but it seems that there is something unexpected, maybe. By design , if the flag "ws_origin" is set as "", there should be no cross-origin check for websocket. But actually if we pass a nil function as CheckOrigin for websocket.Upgradter, the host in the Origin header must not be set or must match the host of the request. So my change is, if "ws_origin" is set as "", ChechOrigin will be a func who always return true. Please kindly correct me if my usage went wrong.

BR, Ye Jianquan