added test for setting nil
added test for setting nil

dvln/objx,onrik/objx,dvln/objx,jmptrader/objx,pki-io/objx,yxd-hde/objx,stretchr/objx,outrightmental/go-objx,yxd-hde/objx,pki-io/objx,jmptrader/objx,onrik/objx,stretchr/objx,outrightmental/go-objx