lua-factory: Ensure that fetches error out on non-UTF-8
lua-factory: Ensure that fetches error out on non-UTF-8

This makes sure that we do not pass binary data as a result
for lua fetch operations.

https://bugzilla.gnome.org/show_bug.cgi?id=728525

GNOME/grilo-plugins,jasuarez/grilo-plugins,vrutkovs/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,vrutkovs/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins