Merge pull request #20296 from vngrs/use_memoization_to_minimize_memory_usage

Use memoization while accessing request headers for minimizing memory usage

actionpack/lib/action_dispatch/http/request.rb

@@ -181,7 +181,7 @@ def head?
     #
     #   request.headers["Content-Type"] # => "text/plain"
     def headers
-      Http::Headers.new(@env)
+      @headers ||= Http::Headers.new(@env)
     end
 
     # Returns a +String+ with the last requested path including their params.