Fix `https://danbooru.donmai.us/artists.json?expires_in=300` failing with
an `'300' is not a valid duration` error. This call pattern is used by the
Translate Pixiv Tags userscript.
Caused by a5ed8c72, which changed the `age:N` metatag to require time
units, but this inadvertently changed the `expires_in` parameter to
require them too.
Using `expires_in` without time units is deprecated and will be removed
in the future.