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"

cc @senny