issue #953: stop using writeHead

object.keys in web-incoming.js results in a non-deterministic ordering of keys, which means that in web-outgoing writeHead might be called before setHeader, which throws an error

Author: jfurler

lib/http-proxy/passes/web-outgoing.js

@@ -137,9 +137,10 @@ module.exports = { // <--
   writeStatusCode: function writeStatusCode(req, res, proxyRes) {
     // From Node.js docs: response.writeHead(statusCode[, statusMessage][, headers])
     if(proxyRes.statusMessage) {
-      res.writeHead(proxyRes.statusCode, proxyRes.statusMessage);
+      res.statusCode = proxyRes.statusCode;
+      res.statusMessage = proxyRes.statusMessage;
     } else {
-      res.writeHead(proxyRes.statusCode);
+      res.statusCode = proxyRes.statusCode;
     }
   }