Make AC::Parameters not inherited from Hash

Following rails/rails#20868 changed how
ActionController::Parameters worked.

This change makes `dump_param_keys` match how the Rails side reads teh
hash keys.

Without this change the right keys don't make it to `Request::Utils` so
the test `test_dasherized_keys_as_xml` was failing because `sub-key`
wasn't being recognized as a key.

test/webservice_test.rb

@@ -13,7 +13,13 @@ def assign_parameters
     def dump_params_keys(hash = params)
       hash.keys.sort.inject("") do |s, k|
         value = hash[k]
-        value = Hash === value ? "(#{dump_params_keys(value)})" : ""
+
+        if value.is_a?(Hash) || value.is_a?(ActionController::Parameters)
+          value = "(#{dump_params_keys(value)})"
+        else
+          value = ""
+        end
+
         s << ", " unless s.empty?
         s << "#{k}#{value}"
       end