Add a user agent if one hasn't been specified
Add a user agent if one hasn't been specified

Signed-off-by: Sam Phippen <e5a09176608998a68778e11d5021c871e8fae93c@googlemail.com>

davidbegin/http,httprb/http.rb,httprb/http,sferik/http,pezra/http.rb,Connorhd/http.rb,moneyadviceservice/http.rb