1.4.0: Default `messages` field to empty array or empty string

Avoids having to do this on the application side.

Also improves test suite.

What's Changed

• Default audit log message property by @rgalanakis in #6

Full Changelog: 1.3.0...1.4.0

1.3.0: Do not reuse audit logs for older transitions

NOTE: Moving releases into GitHub, previously (< 1.3) they were only in Rubygems.

Since most state machines are designed to be idempotent,
it is very possible for a transition to happen multiple times.
Each successful transition should get its own unique audit log entry.
Before this change, we would always update the last audit log
that matched the event/from/to states,
resulting in a loss of history.

This maintains the behavior where failed attempts do get
the audit log for that attempt updated;
new audit logs for each failed transition can add
a huge amount of redundant entries and adds nearly nothing
to actual audit history.

Full Changelog: https://github.com/lithictech/sequel-state-machine/commits/1.3.0