Added check for http GET method and http version when upgrading WebSocket to comply with RFC 6455.

[Marzin-bot]

Added check for http GET method when upgrading WebSocket to comply with RFC 6455.
See https://tools.ietf.org/html/rfc6455

[linusg]

Please also add a check for the version, they're both part of the same requirement in the RFC. ("An HTTP/1.1 or higher GET request")

[Marzin-bot]

http2 is not supported. The WebSocket connection upgrade method for http2 requires the CONNECT method. Therefore, it is best to check the method for each http version in a switch block:

switch (request.head.version) {
        .@"HTTP/1.0" => return false,
        .@"HTTP/1.1" => if (request.head.method != .GET) return false,
 }