use the existing constant rather than the hardcoded string

so tht I don't go totally insane with THsi crazy hash driven
development. why is Everything a hash? Why do people think hashes in
hashes with random keys is a Good API? You can't find things or
deprecate them or control access whatsoever, you just have to hope that
everyone is like "oh, you want to change that? that's cool! we all know
it's hashes so go for it!"

The End.

actionpack/lib/action_dispatch/http/parameters.rb

@@ -25,8 +25,8 @@ def parameters
 
       def path_parameters=(parameters) #:nodoc:
         @symbolized_path_params = nil
-        @env.delete("action_dispatch.request.parameters")
-        @env["action_dispatch.request.path_parameters"] = parameters
+        @env.delete(Routing::RouteSet::PARAMETERS_KEY)
+        @env[Routing::RouteSet::PARAMETERS_KEY] = parameters
       end
 
       # The same as <tt>path_parameters</tt> with explicitly symbolized keys.
@@ -41,7 +41,7 @@ def symbolized_path_parameters
       #
       # See <tt>symbolized_path_parameters</tt> for symbolized keys.
       def path_parameters
-        @env["action_dispatch.request.path_parameters"] ||= {}
+        @env[Routing::RouteSet::PARAMETERS_KEY] ||= {}
       end
 
       def reset_parameters #:nodoc: