Path parameter keys should always be symbols.

If request format is JSON, keys are converted to Strings, so we have to convert them to Symbols.

actionpack/lib/action_controller/test_case.rb

@@ -91,7 +91,7 @@ module ActionController
             end
           end
 
-          path_parameters[key] = value
+          path_parameters[key.to_sym] = value
         end
       end
 

actionpack/test/controller/test_case_test.rb

@@ -613,6 +613,12 @@ XML
     assert_equal({ "bar" => [] }, JSON.load(response.body)["foo"])
   end
 
+  def test_using_as_json_with_path_parameters
+    post :test_params, params: { id: "12345" }, as: :json
+
+    assert_equal("12345", @request.path_parameters[:id])
+  end
+
   def test_mutating_content_type_headers_for_plain_text_files_sets_the_header
     @request.headers["Content-Type"] = "text/plain"
     post :render_body, params: { name: "foo.txt" }