Append values if key already exists in handshake
Append values if key already exists in handshake

Fix for #269

TooTallNate/Java-WebSocket,TooTallNate/Java-WebSocket,leancloud/Java-WebSocket,leancloud/Java-WebSocket,marci4/Java-WebSocket-Dev,leancloud/Java-WebSocket,marci4/Java-WebSocket-Dev