fix UnicodeEncodeError for dbref
fix UnicodeEncodeError for dbref

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