adapted index test to new explain output in pymongo3 and added comment to a possible pymongo3 bug
adapted index test to new explain output in pymongo3 and added comment to a possible pymongo3 bug

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