Send ahoy_message.id as a string

Hi,

I'm running a Rails 3.1.0 application and I try to use ahoy_email, but after send an email I had an error like:

```
undefined method `index' for 1:Fixnum
```

It was occurring in `https://github.com/mikel/mail/blob/2.4.4/lib/mail/encodings.rb#L117` because processor was sending the id as a Fixnum, converting to String, everything works.