Don't crash if it's not possible to attach to property
Don't crash if it's not possible to attach to property

Normally one can attach to properties, but if it is of a type URL then
it crashes hard. This check just drops everything which can't be
attached by `<<`.

G5/microformats2,G5/microformats2,jeena/microformats2,jeena/microformats2,G5/microformats2,jeena/microformats2