fix UnicodeEncodeError for dbref
fix UnicodeEncodeError for dbref

Fix "UnicodeEncodeError: 'ascii' codec can't encode character ..." error in case dbref contains non-ascii characters
reallistic/mongoengine,ProgressivePlanning/mongoengine,closeio/mongoengine,gavincabbage/mongoengine,iandd0824/mongoengine,sourcelair/mongoengine,Davidrjx/mongoengine,WingGao/mongoengine,MakerReduxCorp/mongoengine,khallaghi/mongoengine,blaggacao/mongoengine,ConsumerPhysics/mongoengine,gmacon/mongoengine,losintikfos/mongoengine,yarneo/mongoengine,MakerReduxCorp/mongoengine,9nix00/mongoengine,eli-b/mongoengine,cogniteev/mongoengine,nicolascine/mongoengine,noirbizarre/mongoengine,Multiposting/mongoengine,nathanial/mongoengine,prijindal/mongoengine,RussellLuo/mongoengine,larsbutler/mongoengine,elephanter/mongoengine,touilleMan/mongoengine,Catstyle/mongoengine,muhammad-ammar/mongoengine,jimmyshen/mongoengine,cogniteev/mongoengine,Davidrjx/mongoengine,DavidBord/mongoengine,ms5/mongoengine,MongoEngine/mongoengine,starsirius/mongoengine,MRigal/mongoengine