If MongoGridFS.Find methods returned a real MongoCursor<MongoGridFSFileInfo> instead of IEnumerable<MongoGridFSFileInfo> callers could use cursor methods (e.g. SetSortOrder, SetSkip and SetLimit to do pagination).

This change should be backward compatible because MongoCursor<MongoGridFSFileInfo> implements IEnumerable<MongoGridFSFileInfo>.