There was an issue when looking up the default handler
for a type where a supertype was listed in defaults.list.
We would pick the default for the parent type even if
there was a handler for the more specific type.

In the case of the new-style defaults marking (
"Default Applications" in mimeapps.list) we were already
checking for a more specific handler befor using a default,
but we also need to do a similar check for the defaults.list
case.

https://bugzilla.gnome.org/show_bug.cgi?id=678944