Don't access res._headers directly when helpers exist

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

Use the documented helper functions instead when they are available.

lib/context.js

@@ -116,7 +116,12 @@ var proto = module.exports = {
     }
 
     // unset all headers, and set those specified
-    this.res._headers = {};
+    if (this.res.getHeaderNames) {
+      this.res.getHeaderNames().forEach(function(name) { this.removeHeader(name) }, this.res);
+    } else {
+      this.res._headers = {}; // Node < 8
+    }
+
     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/response/header.js

@@ -10,6 +10,16 @@ describe('res.header', function(){
     res.header.should.eql({ 'x-foo': 'bar' });
   })
 
+  it('should use res.getHeades 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();