[feature] Add option to support late addition of headers

This supports the use-case where headers need to be added that depend on
the socket connection (e.g. for TLS channel binding).

Author: mvduin

lib/websocket.js

@@ -989,7 +989,11 @@ function initAsClient(websocket, address, protocols, options) {
     });
   });
 
-  req.end();
+  if (typeof opts.finishRequest === 'function') {
+    opts.finishRequest(req, websocket);
+  } else {
+    req.end();
+  }
 }
 
 /**