Fix _transform_update to accept unicode fields
Fix _transform_update to accept unicode fields

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