Fix AuthZ plugins headers change issue

This fix tries to address the issue raised in 25927 where
the HTTP headers have been chaged when AUthZ plugin is in
place.

This issue is that in `FlushAll` (`pkg/authorization/response.go`),
the headers have been written (with `WriteHeader`) before all the
headers have bee copied.

This fix fixes the issue by placing `WriteHeader` after.

A test has been added to cover the changes.`

This fix fixes 25927

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
(cherry picked from commit 9cb8fb6ea03fcd78010ce7dd33585d96cd73e38c)
Signed-off-by: Victor Vieux <vieux@docker.com>

integration-cli/docker_cli_authz_unix_test.go

@@ -443,6 +443,25 @@ func (s *DockerAuthzSuite) TestAuthZPluginEnsureLoadImportWorking(c *check.C) {
 	c.Assert(err, check.IsNil, check.Commentf(out))
 }
 
+func (s *DockerAuthzSuite) TestAuthZPluginHeader(c *check.C) {
+	c.Assert(s.d.Start("--debug", "--authorization-plugin="+testAuthZPlugin), check.IsNil)
+	s.ctrl.reqRes.Allow = true
+	s.ctrl.resRes.Allow = true
+	c.Assert(s.d.LoadBusybox(), check.IsNil)
+
+	daemonURL, err := url.Parse(s.d.sock())
+
+	conn, err := net.DialTimeout(daemonURL.Scheme, daemonURL.Path, time.Second*10)
+	c.Assert(err, check.IsNil)
+	client := httputil.NewClientConn(conn, nil)
+	req, err := http.NewRequest("GET", "/version", nil)
+	c.Assert(err, check.IsNil)
+	resp, err := client.Do(req)
+
+	c.Assert(err, check.IsNil)
+	c.Assert(resp.Header["Content-Type"][0], checker.Equals, "application/json")
+}
+
 // assertURIRecorded verifies that the given URI was sent and recorded in the authz plugin
 func assertURIRecorded(c *check.C, uris []string, uri string) {
 	var found bool

pkg/authorization/response.go

@@ -175,11 +175,6 @@ func (rm *responseModifier) Flush() {
 
 // FlushAll flushes all data to the HTTP response
 func (rm *responseModifier) FlushAll() error {
-	// Copy the status code
-	if rm.statusCode > 0 {
-		rm.rw.WriteHeader(rm.statusCode)
-	}
-
 	// Copy the header
 	for k, vv := range rm.header {
 		for _, v := range vv {
@@ -187,6 +182,13 @@ func (rm *responseModifier) FlushAll() error {
 		}
 	}
 
+	// Copy the status code
+	// Also WriteHeader needs to be done after all the headers
+	// have been copied (above).
+	if rm.statusCode > 0 {
+		rm.rw.WriteHeader(rm.statusCode)
+	}
+
 	var err error
 	if len(rm.body) > 0 {
 		// Write body