Compare state directly in specs.

Previously this test used the invalid? method definition from state
machine which conflicted with a method from ActiveModel. Instead of
dealing with that issue, we will compare the state directly to what it
should be rather than relying upon the magic methods defined by
state_machine.

Fixes #5
Merge pull request #8 from gmacdougall/2-2-state-machine-method-conflict-fix

[2-2-stable] Compare state directly in specs.