Add a section to the config file for HTTP headers to add to responses

The example configuration files add X-Content-Type-Options: nosniff.

Add coverage in existing registry/handlers unit tests.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

cmd/registry/config-example.yml

@@ -9,3 +9,5 @@ storage:
         rootdirectory: /var/lib/registry
 http:
     addr: :5000
+    headers:
+        X-Content-Type-Options: [nosniff]