Prevent headers to be sent twice

Author: samalba

lib/node-http-proxy/http-proxy.js

@@ -333,10 +333,12 @@ HttpProxy.prototype.proxyRequest = function (req, res, buffer) {
     }
 
     // Set the headers of the client response
-    Object.keys(response.headers).forEach(function (key) {
-      res.setHeader(key, response.headers[key]);
-    });
-    res.writeHead(response.statusCode);
+    if (res.sentHeaders !== true) {
+        Object.keys(response.headers).forEach(function (key) {
+            res.setHeader(key, response.headers[key]);
+        });
+        res.writeHead(response.statusCode);
+    }
 
     function ondata(chunk) {
       if (res.writable) {