gio: Add regression test for double array encoding

https://bugzilla.gnome.org/show_bug.cgi?id=732754
gio: Fix regression encoding an array of doubles

Take the simple slow path in this case. Encoding a double
takes special precautions as you can see in append_value_to_blob()
and friends.

https://bugzilla.gnome.org/show_bug.cgi?id=732754