fix: res.flushHeaders() (#795)

* fix: res.flushHeaders()

* remove arg to flush headers

* fix tests for node v4 and v5

lib/response.js

@@ -534,6 +534,6 @@ module.exports = {
    * Flush any set headers, and begin the body
    */
   flushHeaders() {
-    this.res.writeHead(this.res.statusCode);
+    this.res.flushHeaders();
   }
 };

test/response/flushHeaders.js

@@ -4,6 +4,7 @@
 const request = require('supertest');
 const assert = require('assert');
 const Koa = require('../..');
+const http = require('http');
 
 describe('ctx.flushHeaders()', () => {
   it('should set headersSent', done => {
@@ -95,4 +96,43 @@ describe('ctx.flushHeaders()', () => {
         done(err);
       });
   });
+
+  it('should flush headers first and delay to send data', done => {
+    const PassThrough = require('stream').PassThrough;
+    const app = new Koa();
+
+    app.use(ctx => {
+      ctx.type = 'json';
+      ctx.status = 200;
+      ctx.headers['Link'] = '</css/mycss.css>; as=style; rel=preload, <https://img.craftflair.com>; rel=preconnect; crossorigin';
+      const stream = ctx.body = new PassThrough();
+      ctx.flushHeaders();
+
+      setTimeout(() => {
+        stream.end(JSON.stringify({ message: 'hello!' }));
+      }, 10000);
+    });
+
+    app.listen(function(err){
+      if (err) return done(err);
+
+      const port = this.address().port;
+
+      http.request({
+        port
+      })
+      .on('response', res => {
+        const onData = () => done(new Error('boom'));
+        res.on('data', onData);
+
+        // shouldn't receive any data for a while
+        setTimeout(() => {
+          res.removeListener('data', onData);
+          done();
+        }, 1000);
+      })
+      .on('error', done)
+      .end();
+    });
+  });
 });