Add 'proxyResponse' event so observer can modify response headers or abort response.

Author: colinmollenhour

Diff for: lib/node-http-proxy/http-proxy.js

@@ -301,6 +301,13 @@ HttpProxy.prototype.proxyRequest = function (req, res, buffer) {
       }
     });
 
+    // Allow observer to modify headers or abort response
+    try { req.emit('proxyResponse', req, res, response) }
+    catch (ex) {
+      errState = true;
+      return;
+    }
+
     // Set the headers of the client response
     Object.keys(response.headers).forEach(function (key) {
       res.setHeader(key, response.headers[key]);