added failing spec to show bug when undoing a change on an embed_one embedded doc
added failing spec to show bug when undoing a change on an embed_one embedded doc

sivagollapalli/mongoid-history,mateuspontes/mongoid-history,Yakrware/mongoid-history,stephenhmarsh/mongoid-history,mongoid/mongoid-history,aq1018/mongoid-history,getaroom/mongoid-history,dblock/mongoid-history