Avoid overriding headers set in onresponse

Currently, overriding a header from the target response in the source
response requires doing it in both ondata_response (when receiving the first
chunk of data) and in onend_response (for when the response doesn't contain
any data), after verifying that res.headersSent is false. That's clumsy at
best.

By filtering the copied headers with those already present in the source
response, it's possible to set them in onresponse once and for all.

lib/plugins-middleware.js

@@ -364,7 +364,9 @@ function handleTargetResponse(targetRequest, targetResponse, options, cb) {
             Object.keys(targetResponse.headers).forEach(function(header) {
                 // skip setting the 'connection: keep-alive' header
                 // setting it causes gateway to not accept any more connections
-                if (header !== 'connection') {
+                // Headers that have been set in onresponse are also skipped to let the plugins override those from the
+                // target response.
+                if (header !== 'connection' && !sourceResponse.hasHeader(header)) {
                     sourceResponse.setHeader(header, targetResponse.headers[header]);
                 }
             });

tests/plugin-lifecycle-tests.js

@@ -3,6 +3,7 @@ var assert = require('assert');
 var gatewayService = require('../index');
 var serverFactory = require('./hello_rest/index');
 var TestPlugin = require('./testPlugin');
+var TestResponseHeaderPlugin = require('./testResponseHeaderPlugin');
 var request = require('request');
 var should = require('should')
 
@@ -301,6 +302,20 @@ describe('test lifecycle events', function() {
       });
     });
   });
+
+  it('should keep the header set on the source response', function(done) {
+    this.timeout(20000);
+    var testPlugin = TestResponseHeaderPlugin();
+    var handler = testPlugin.init();
+    gateway.addPlugin('test', function test() { return handler });
+    gateway.start(function(err) {
+      assert(!err, err);
+      request({ method: "GET", url: 'http://localhost:' + gatewayPort + '/v1/echo/get' }, (err, r) => {
+        assert.equal(r.headers['content-type'], 'application/octet-stream');
+        done();
+      });
+    });
+  });
 });
 
 function _findHeaders(headers, expectedHeaders) {

tests/testResponseHeaderPlugin.js

@@ -0,0 +1,17 @@
+'use strict';
+
+const TestResponseHeaderPlugin = function () {
+};
+
+module.exports = function () {
+  return new TestResponseHeaderPlugin();
+};
+
+TestResponseHeaderPlugin.prototype.init = function myPlugin1() {
+  return {
+    onresponse: function (sourceReq, sourceRes, targetRes, data, next) {
+      sourceRes.setHeader('content-type', 'application/octet-stream');
+      next();
+    }
+  };
+};