Fix "Can't set headers after they are sent" errors

This PR tries to fix "Can't set headers after they are sent" errors.
That are a lot of situations where this error can occurs. In my case, it is happening because I have others middlewares (in an expressjs application that tries to proxy requests). Some of those middlewares (like [passportjs](http://passportjs.org/), or [cors](https://www.npmjs.com/package/cors)) can run ```res.end()``` and when the proxy receive a response, it is already finished.
So, it is necessary to test if we can write on the user response when the proxy response is ready.
I think it could also fix http-party#930, http-party#1168, http-party#908

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

@@ -162,16 +162,22 @@ module.exports = {
 
     proxyReq.on('response', function(proxyRes) {
       if(server) { server.emit('proxyRes', proxyRes, req, res); }
-      for(var i=0; i < web_o.length; i++) {
-        if(web_o[i](req, res, proxyRes, options)) { break; }
+      if (!res.headersSent) {
+        for(var i=0; i < web_o.length; i++) {
+          if(web_o[i](req, res, proxyRes, options)) { break; }
+        }
       }
 
       // Allow us to listen when the proxy has completed
-      proxyRes.on('end', function () {
-        server.emit('end', req, res, proxyRes);
-      });
-
-      proxyRes.pipe(res);
+      if (!res.finished) {
+        proxyRes.on('end', function () {
+          server.emit('end', req, res, proxyRes);
+        });
+        proxyRes.pipe(res);
+      }
+      else {
+          server.emit('end', req, res, proxyRes);
+      }
     });
 
     //proxyReq.end();