adds test for userInfo in FetchableRecord.databaseJSONDecoder()
adds test for userInfo in FetchableRecord.databaseJSONDecoder()

Making sure that databaseDecodingUserInfo is being applied to FetchableRecord.databaseJSONDecoder()

This tries to cover some of the changes in PR groue#958

groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift