code: public WebSocketProtocolHandshakeHandler addExtension(ExtensionHandshake extension) {
          if (extension != null) {
              for (Handshake handshake : handshakes) {
                  handshake.addExtension(extension);
              }
          }
          return this;
      }
docs: Add a new WebSocket Extension into the handshakes defined in this handler.
  
  @param extension a new {@code ExtensionHandshake} instance
  @return          current handler