fixed bug in handling (secure) websocket connection

Apparently the binary string "Websocket" does show up at least
in another alternative. The concerning request was sent from a 
Windows 8 machine, using Internet Explorer 10. 
Maybe one should consider something like:
----
 {WS, _Req2} -> case string:to_lower(binary_to_list(WS)) of
                       "websocket" -> {upgrade, protocol, cowboy_http_websocket};
                       _ -> io:format("sorry, unknown request~n")
                   end
----
Just in case, some fancy browser sends <<"wEBsocKEt">>