Added regression test for issue with unset and pop
Added regression test for issue with unset and pop

Closes #118

ms5/mongoengine,yarneo/mongoengine,depop/mongoengine-stripped,iandd0824/mongoengine,blaggacao/mongoengine,hmarr/mongoengine,elephanter/mongoengine,ConsumerPhysics/mongoengine,Davidrjx/mongoengine,touilleMan/mongoengine,noirbizarre/mongoengine,sourcelair/mongoengine,starsirius/mongoengine,ProgressivePlanning/mongoengine,nathanial/mongoengine,ContextLogic/mongoengine,Catstyle/mongoengine,ContextLogic/mongoengine,cogniteev/mongoengine,RussellLuo/mongoengine,DavidBord/mongoengine,MakerReduxCorp/mongoengine,eli-b/mongoengine,jimmyshen/mongoengine,muhammad-ammar/mongoengine,nicolascine/mongoengine,closeio/mongoengine,prijindal/mongoengine,gavincabbage/mongoengine,conversocial/mongoengine,newvem/mongoengine,cogniteev/mongoengine,reallistic/mongoengine,MakerReduxCorp/mongoengine,Davidrjx/mongoengine,losintikfos/mongoengine,MongoEngine/mongoengine,aszwemin/mongoengine,gmacon/mongoengine,WingGao/mongoengine,khallaghi/mongoengine,KarimAllah/mongoengine,colinhowe/mongoengine,larsbutler/mongoengine,9nix00/mongoengine,MRigal/mongoengine,Multiposting/mongoengine