[ws] add options to transform client and server streams

Author: GertSallaerts

README.md

@@ -373,6 +373,8 @@ proxyServer.listen(8015);
      }
      ```
 *  **headers**: object with extra headers to be added to target requests.
+*  **wsClientTransformStream**: if set, data from client websocket will be piped through this stream before being sent to the server, allowing you to influence the request data
+*  **wsServerTransformStream**: if set, data from server websocket will be piped through this stream before being sent to the client, allowing you to influence the response data
 *  **proxyTimeout**: timeout (in millis) for outgoing proxy requests
 *  **timeout**: timeout (in millis) for incoming requests
 *  **followRedirects**: true/false, Default: false - specify whether you want to follow redirects

lib/http-proxy/passes/ws-incoming.js

@@ -142,7 +142,21 @@ module.exports = {
       //
       socket.write(createHttpHeader('HTTP/1.1 101 Switching Protocols', proxyRes.headers));
 
-      proxySocket.pipe(socket).pipe(proxySocket);
+      let proxyStream = proxySocket;
+
+      if (options.wsServerTransformStream) {
+        options.wsServerTransformStream.on('error', onOutgoingError);
+        proxyStream = proxyStream.pipe(options.wsServerTransformStream);
+      }
+
+      proxyStream = proxyStream.pipe(socket);
+
+      if (options.wsClientTransformStream) {
+        options.wsClientTransformStream.on('error', onOutgoingError);
+        proxyStream = proxyStream.pipe(options.wsClientTransformStream);
+      }
+
+      proxyStream.pipe(proxySocket);
 
       server.emit('open', proxySocket);
       server.emit('proxySocket', proxySocket);  //DEPRECATED.