Fix #7 for including blanks in ids.

It turns out that ActiveRecord rejects blanks before setting the ids. (It also allows setting to a single id, which I added a test for too.) https://github.com/rails/rails/blob/b8302bcfdaec2a9e7658262d6feeb535c572922d/activerecord/lib/active_record/associations/collection_association.rb#L59