Make it possible to set the user agent for websockets

[Mathy Vanvoorden (MathyV)]

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

[Microsoft Contribution License Agreements (msftclas)]

All CLA requirements met.

[Gareth Sylvester-Bradley (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)?

[Mathy Vanvoorden (MathyV)]

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

[Robert Schumacher (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 Gareth Sylvester-Bradley (@garethsb-sony)'s snippet work (as one would expect) but also provides the more friendly set_user_agent() API.