Fix to get specs running, and use float for comparison to make it easier...

... to read the failure.

@huoxito Out of the box I'm getting a failure:

```
Failures:

  1) Spree::AdyenRedirectController sets payment attributes properly
     Failure/Error: expect(payment.amount.to_f).to eq order.total.to_f
       
       expected: 50.0
            got: 150.0
       
       (compared using ==)
     # ./spec/controllers/spree/adyen_redirect_controller_spec.rb:34:in `block (2 levels) in <module:Spree>'

Finished in 3.11 seconds
19 examples, 1 failure, 2 pending
```

I'm also experiencing same issue expecting 50 and getting a result of 150 in my drop ship extension while working on 2.2.x upgrade from 2.1.x so probably related to a change in Spree itself I suspect.  Haven't had time to dig into it yet, but this at least makes it easy for me to start running specs here.

I'm beginning to investigate issue with guest checkout not working with this payment method, but works when logged in as a user.  So any ideas / thoughts on that would be great @huoxito

/cc @bdq commit access please?