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 #930, #1168, #908

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

@@ -170,21 +170,23 @@ module.exports = {
     proxyReq.on('response', function(proxyRes) {
       if(server) { server.emit('proxyRes', proxyRes, req, res); }
 
-      // Allow us to listen when the proxy has completed
-      proxyRes.on('end', function () {
-        server.emit('end', req, res, proxyRes);
-      });
-
-      if(!options.selfHandleResponse) {
+      if(!res.headersSent && !options.selfHandleResponse) {
         for(var i=0; i < web_o.length; i++) {
           if(web_o[i](req, res, proxyRes, options)) { break; }
         }
-
-        proxyRes.pipe(res);
       }
-    });
 
-    //proxyReq.end();
+      if (!res.finished) {
+        // Allow us to listen when the proxy has completed
+        proxyRes.on('end', function () {
+          if (server) server.emit('end', req, res, proxyRes);
+        });
+        // We do this separately since we are also checking for res.finished
+        if (!options.selfHandleResponse) proxyRes.pipe(res);
+      } else {
+        if (server) server.emit('end', req, res, proxyRes);
+      }
+    });
   }
 
 };