If a user presses the 'Register' button too quickly after attempting
to solve the reCAPTCHA, or the reCAPTCHA is not solved at all, the
user would experience a 500 error. Now, the case is properly
handled and the user will be sent back to the registration page
with a clear error message and can try again.
- Look for a `change` in its size rather than asserting against an
actual size.
- This previously failed because another spec had an email in
`ActionMailer::Base.deliveries`, which failed this `be_nil` assertion.
