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 #7 from gmacdougall/state-machine-method-conflict-fix

Compare state directly in specs.