Increase test coverage of DatabaseCoder.swift
Increase test coverage of DatabaseCoder.swift

Test failure cases.

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