Fix specs for mail interceptor

It solves this kind of errors running specs:

```
Spree::OrderMailer#deliver when intercept_mode is not provided should not modify the recipient
     Failure/Error: message.deliver
     ArgumentError:
       A sender (Return-Path, Sender or From) required to send a message
     # ./spec/lib/mail_interceptor_spec.rb:72:in `block (4 levels) in <top (required)>'
```

thanks to @mtylty for the original solution.