use get / set header to avoid depending on the `env` ivar

actionpack/lib/action_dispatch/http/parameters.rb

@@ -8,20 +8,23 @@ module Parameters
 
       # Returns both GET and POST \parameters in a single hash.
       def parameters
-        @env["action_dispatch.request.parameters"] ||= begin
-          params = begin
-            request_parameters.merge(query_parameters)
-          rescue EOFError
-            query_parameters.dup
-          end
-          params.merge!(path_parameters)
-        end
+        params = get_header("action_dispatch.request.parameters")
+        return params if params
+
+        params = begin
+                   request_parameters.merge(query_parameters)
+                 rescue EOFError
+                   query_parameters.dup
+                 end
+        params.merge!(path_parameters)
+        set_header("action_dispatch.request.parameters", params)
+        params
       end
       alias :params :parameters
 
       def path_parameters=(parameters) #:nodoc:
-        @env.delete('action_dispatch.request.parameters')
-        @env[PARAMETERS_KEY] = parameters
+        delete_header('action_dispatch.request.parameters')
+        set_header PARAMETERS_KEY, parameters
       end
 
       # Returns a hash with the \parameters used to form the \path of the request.
@@ -29,7 +32,7 @@ def path_parameters=(parameters) #:nodoc:
       #
       #   {'action' => 'my_action', 'controller' => 'my_controller'}
       def path_parameters
-        @env[PARAMETERS_KEY] ||= {}
+        get_header(PARAMETERS_KEY) || {}
       end
 
     private