Request#check_method no longer breaks when :en is not available locale

Request#check_method would use to_sentence(locale: :en), which breaks when
I18n.available_locales does not include :en and
I18n.enforce_available_locales is true (default).

Inlined to_sentence functionality to solve this.
Merge pull request #17221 from StefanH/fix-unknown-http-method-message

Invalid HTTP method will raise I18n::InvalidLocale if :en is not a valid locale.