Add serialized `data` attribute to Ahoy::Message model

Solves the same problem as https://github.com/ankane/ahoy_email/pull/8/files, but my approach seems more idiomatic for this lib.

It's a little tough developing without tests -- @ankane, is there any reason there's no simple test suite for this gem? I'm assuming it's just because of time & bandwidth constraints, something I totally understand :)