mail_view for human — and also for automatic — testing

I've come to use a nice gem called mail_view for visual testing of email templates in Rails.
I think it is totally legit and desirable to re-use that code in the views testing suite, so here we go:
(provided that your MailView's subclass is MailPreview, put this in ./spec/views/mail_spec.rb)

This just checks for the lack of exceptions and any content, but it's a starting point.

p.s. A nice complementary pull request of mine is here.