use public API to fetch the parameter filter

now the parameter filter doesn't need to know about the env hash in
these two methds.

actionpack/lib/action_dispatch/http/filter_parameters.rb

@@ -50,13 +50,13 @@ def filtered_path
     protected
 
       def parameter_filter
-        parameter_filter_for @env.fetch("action_dispatch.parameter_filter") {
+        parameter_filter_for get_header("action_dispatch.parameter_filter") {
           return NULL_PARAM_FILTER
         }
       end
 
       def env_filter
-        user_key = @env.fetch("action_dispatch.parameter_filter") {
+        user_key = get_header("action_dispatch.parameter_filter") {
           return NULL_ENV_FILTER
         }
         parameter_filter_for(Array(user_key) + ENV_MATCH)