refactors append_metadata so that it applies proxy term behavior more extensibly
refactors append_metadata so that it applies proxy term behavior more extensibly

samvera/hyrax,jeremyf/sufia,jeremyf/sufia,samvera/hyrax,ndlib/scholarsphere,zhang4952/scholarsphere,zhang4952/scholarsphere,zhang4952/scholarsphere,samvera/hyrax,samvera/hyrax,ndlib/scholarsphere,zhang4952/scholarsphere,ndlib/scholarsphere