$setOnInsert should not create an empty parent object when it should be a no op

        case Mod::RENAME_FROM:

        case Mod::RENAME_TO:

            if (modState.dontApply) {

                return;

            }

c = db.c;

c.drop();

c.save( { a:1 } );

c.update( {}, { $setOnInsert:{ 'b.c':1 }, $set:{ d:1 } } );

printjson( c.findOne() );

{

	"_id" : ObjectId("5127d71b69c410f31b5f2ef1"),

	"a" : 1,

	"b" : {          // <-- There should not be a 'b' field here.

	},

	"d" : 1

}