Tests for inactive account detection

[nbudin]

Pull request #10 is a useful feature but lacks tests, and the author has let me know that he's not going to be able to write the tests himself. We can probably do this with the mock XML that Peng from Google sent us:

<error xmlns="http://checkout.google.com/schema/2" serial-number="3c394432-8270-411b-9239-98c2c499f87f">
  <error-message>Seller Account xxxxxxxxx is not active.</error-message>
</error>