Make it possible to set the user agent for websockets

[MathyV]

Since I don't have a box to test winrt implementation I can't add/test that unfortunately

[msftclas]

All CLA requirements met.

[garethsb]

I'm not sure, but can this be done already with my_config.headers().add(web::http::header_names::user_agent, my_user_agent)?

[MathyV]

I tried but websocketpp overwrites it as far as I can tell if you don't specify it explicitly.

[ras0219-msft]

Thanks for the PR!

I changed the new set API to use the headers structure to pass the user agent in -- this makes @garethsb-sony's snippet work (as one would expect) but also provides the more friendly set_user_agent() API.