metaprogram the HTTP_X_REQUEST_ID method

we're already doing this with a bunch of other header data.

actionpack/lib/action_dispatch/http/request.rb

@@ -20,8 +20,6 @@ class Request < Rack::Request
     include ActionDispatch::Http::FilterParameters
     include ActionDispatch::Http::URL
 
-    HTTP_X_REQUEST_ID = "HTTP_X_REQUEST_ID".freeze # :nodoc:
-
     autoload :Session, 'action_dispatch/request/session'
     autoload :Utils,   'action_dispatch/request/utils'
 
@@ -36,6 +34,7 @@ class Request < Rack::Request
         HTTP_ACCEPT_LANGUAGE HTTP_CACHE_CONTROL HTTP_FROM
         HTTP_NEGOTIATE HTTP_PRAGMA HTTP_CLIENT_IP
         HTTP_X_FORWARDED_FOR HTTP_VERSION
+        HTTP_X_REQUEST_ID
         ].freeze
 
     ENV_METHODS.each do |env|
@@ -269,10 +268,6 @@ def request_id=(id) # :nodoc:
 
     alias_method :uuid, :request_id
 
-    def x_request_id # :nodoc:
-      get_header(HTTP_X_REQUEST_ID)
-    end
-
     # Returns the lowercase name of the HTTP server software.
     def server_software
       (get_header('SERVER_SOFTWARE') && /^([a-zA-Z]+)/ =~ get_header('SERVER_SOFTWARE')) ? $1.downcase : nil