[enhancement] add ability to remove default headers

[zxsleebu]

default headers on websocket client connection like "User-Agent" and "Origin" are very annoying.
please, add an ability to remove automatic addition of these headers.
it would be nice if they could be removed, because in my case if these headers are present, server drops the connection

[zxsleebu]

that's what i've done to get ixwebsocket work with riot games webserver. zxsleebu@7aa7dcd

[bsergean]

Hey would you mind making a PR for that ?

We could drop userAgent, but droping Origin sounds a bit more problematic. We'd need some option for that.