UTF-8 and ASCII-8BIT

I'm writing a feature that uses your gem and I'm so close to finishing, but when the client sent over a bach of emails to display, I can't convert a bunch of them due to this:

```
/Users/username/.rvm/gems/ruby-2.3.3/gems/ruby-msg-1.5.2/lib/mapi/mime.rb:109:in `join': incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
```