Add with_default_charset helper.
Merge pull request #15381 from zuhao/refactor_actionpack_content_type_test

Add with_default_charset helper.
Move I18n.locale setting into setup and teardown.
Merge pull request #15383 from zuhao/refactor_actionpack_localized_templates_test

Move I18n.locale setting into setup and teardown.
Add and remove renderer inside the test to prevent leak.
Merge pull request #15385 from zuhao/refactor_actionpack_render_other_test

Add and remove renderer inside the test to prevent leak.