Added fix in FongoDBCollection and test in FongoTest for find if only _i...

Added fix in FongoDBCollection and test in FongoTest for find if only _id is excluded. For example, with db {_id:1,a:1}{_id:2,a:2} a find({},{_id:0}) gives {a:1}{a:2} in mongo, but (without fix) {}{} in fongo.