Motivation:
Currently we passing custom websocket handshaker response headers to a `WebSocketServerHandshaker` but they can contain a reserved headers (e.g. Connection, Upgrade, Sec-Websocket-Accept) what lead to duplication because we use response.headers().add(..) instead of response.headers().set(..).
Modification:
In each `WebSocketServerHandshaker00`, ... `WebSocketServerHandshaker13` implementation replace the method add(..) to set(..) for reserved response headers.
Result:
Less error-prone