Add a test case of invalid transition with not-bang method
Add a test case of invalid transition with not-bang method

At least it is better to check status is not changed by
invalid transition with not-bang method.
Also I think returning `false` when invalid transition is called with
not-bang method (e.g. `bug.assign`) is intended API behavior, so adding
`assert_equal false, bug.assign`.

amatsuda/stateful_enum,amatsuda/stateful_enum