Only enable HSTS header for HTTPS and port 443

Closes https://github.com/gitlabhq/gitlabhq/issues/9449

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.13.0 (unreleased)
+  - Only enable HSTS header for HTTPS and port 443 (Stan Hu)
   - Fix user autocomplete for unauthenticated users accessing public projects (Stan Hu)
   - Fix redirection to home page URL for unauthorized users (Daniel Gerhardt)
   - Add branch switching support for graphs (Daniel Gerhardt)

app/controllers/application_controller.rb

@@ -183,7 +183,10 @@ class ApplicationController < ActionController::Base
     headers['X-XSS-Protection'] = '1; mode=block'
     headers['X-UA-Compatible'] = 'IE=edge'
     headers['X-Content-Type-Options'] = 'nosniff'
-    headers['Strict-Transport-Security'] = 'max-age=31536000' if Gitlab.config.gitlab.https
+    # Enabling HSTS for non-standard ports would send clients to the wrong port
+    if Gitlab.config.gitlab.https and Gitlab.config.gitlab.port == 443
+      headers['Strict-Transport-Security'] = 'max-age=31536000'
+    end
   end
 
   def add_gon_variables