test: add tests for add/remove header after sent

This change adds tests to make sure an Error is thrown if a header
is added or removed after they are sent.

PR-URL: https://github.com/nodejs/node/pull/8682
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

test/parallel/test-http-response-add-header-after-sent.js

@@ -0,0 +1,21 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const server = http.createServer((req, res) => {
+  assert.doesNotThrow(() => {
+    res.setHeader('header1', 1);
+  });
+  res.write('abc');
+  assert.throws(() => {
+    res.setHeader('header2', 2);
+  }, /Can't set headers after they are sent\./);
+  res.end();
+});
+
+server.listen(0, () => {
+  http.get({port: server.address().port}, () => {
+    server.close();
+  });
+});

test/parallel/test-http-response-remove-header-after-sent.js

@@ -0,0 +1,21 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const server = http.createServer((req, res) => {
+  assert.doesNotThrow(() => {
+    res.removeHeader('header1', 1);
+  });
+  res.write('abc');
+  assert.throws(() => {
+    res.removeHeader('header2', 2);
+  }, /Can't remove headers after they are sent/);
+  res.end();
+});
+
+server.listen(0, () => {
+  http.get({port: server.address().port}, () => {
+    server.close();
+  });
+});