Fix SameSite=None incompatible check not working

!40667 (merged) didn't actually work because we need to read the Rack HTTP_USER_AGENT entry from the request headers instead of the response headers.

Relates to #241785 (closed)

spec/lib/gitlab/middleware/same_site_cookies_spec.rb

@@ -18,7 +18,7 @@ def initialize(cookies)
       def call(env)
         [
           200,
-          { 'Set-Cookie' => cookies }.compact,
+          { 'Set-Cookie' => cookies },
           ['OK']
         ]
       end