fewer calls to `env`

We don't want to directly access the env hash

actionpack/lib/action_dispatch/http/cache.rb

@@ -8,13 +8,13 @@ module Request
         HTTP_IF_NONE_MATCH     = 'HTTP_IF_NONE_MATCH'.freeze
 
         def if_modified_since
-          if since = env[HTTP_IF_MODIFIED_SINCE]
+          if since = get_header(HTTP_IF_MODIFIED_SINCE)
             Time.rfc2822(since) rescue nil
           end
         end
 
         def if_none_match
-          env[HTTP_IF_NONE_MATCH]
+          get_header HTTP_IF_NONE_MATCH
         end
 
         def if_none_match_etags