Properly fixed indexing on _id for covered indexes
Properly fixed indexing on _id for covered indexes

refs  #4

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