Unwrap "(as)"-typed `mime_types` value coming from Mutter
requested to merge mikedld/xdg-desktop-portal-gnome:bugfix/136-clipboard-selownch-mimetypes-type into main
Clipboard XDP spec says mime_types is "as" while value coming from
Mutter is "(as)" which cannot be forwarded as-is. Be forward-compatible
and only unwrap the value if it's wrapped.
Fixes #136.