response: don't access res._headers directly when helpers exist

res._headers is considered internal to node itself. Its value will
change in a backwards incompatible way in the future node releases.

Use the documented helper functions instead when they are available.

lib/context.js

@@ -115,8 +115,14 @@ var proto = module.exports = {
       return;
     }
 
-    // unset all headers, and set those specified
-    this.res._headers = {};
+    // first unset all headers
+    if (this.res.getHeaderNames) {
+      this.res.getHeaderNames().forEach(function(name) { this.removeHeader(name) }, this.res);
+    } else {
+      this.res._headers = {}; // Node < 8
+    }
+
+    // then set those specified
     this.set(err.headers);
 
     // force text/plain

lib/response.js

@@ -44,7 +44,9 @@ module.exports = {
    */
 
   get header() {
-    return this.res._headers || {};
+    return this.res.getHeaders
+      ? this.res.getHeaders()
+      : this.res._headers || {};  // Node < 8
   },
 
   /**

test/context/onerror.js

@@ -1,8 +1,10 @@
 
 'use strict';
 
+var assert = require('assert');
 var request = require('supertest');
 var koa = require('../..');
+var context = require('../context');
 
 describe('ctx.onerror(err)', function(){
   it('should respond', function(done){
@@ -147,4 +149,21 @@ describe('ctx.onerror(err)', function(){
       .expect('Internal Server Error', done);
     })
   })
+
+  it('should use res.getHeaderNames when available', function(){
+    var removed = 0;
+    var ctx = context();
+
+    ctx.app.emit = function() {};
+    ctx.res = {
+      getHeaderNames: function() { return ['content-type', 'content-length'] },
+      removeHeader: function() { removed++ },
+      end: function() {},
+      emit: function() {}
+    };
+
+    ctx.onerror(new Error('error'));
+
+    assert.equal(removed, 2);
+  })
 })

test/response/header.js

@@ -10,6 +10,16 @@ describe('res.header', function(){
     res.header.should.eql({ 'x-foo': 'bar' });
   })
 
+  it('should use res.getHeaders helper when it is available', function(){
+    var res = response(null, {
+      _headers: {},
+      getHeaders: function(){
+        return { 'x-foo': 'baz' }
+      }
+    });
+    res.header.should.eql({ 'x-foo': 'baz' });
+  })
+
   describe('when res._headers not present', function (){
     it('should return empty object', function (){
       var res = response();