Restore I18n.locale to default after test.

If i18n_with_controller_test.rb were to run first, the I18n.locale will
be changed to :de, and the following tests in base_test.rb will fail:

"subject gets default from I18n"
“default subject can have interpolations”
"translations are scoped properly"
"implicit multipart with default locale"
Merge pull request #15548 from zuhao/refactor_actionmailer_i18n_with_controller_test

Restore I18n.locale to default after test.