Encoding::CompatibilityError

Original error message:
```
/Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/lib/mime.rb:109:in `join': incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/lib/mime.rb:109:in `to_s'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:104:in `block in process_message'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/open-uri.rb:36:in `open'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/open-uri.rb:36:in `open'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:125:in `process_message'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/lib/mapi/msg.rb:344:in `open'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:46:in `block in each_message'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:30:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:30:in `each_message'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:52:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:171:in `mapitool'
	from /Library/Ruby/Gems/2.0.0/gems/ruby-msg-1.5.1/bin/mapitool:174:in `<top (required)>'
	from /usr/bin/mapitool:23:in `load'
	from /usr/bin/mapitool:23:in `<main>'
```

I ran through your how-to in IRB and traced it to line 109 of `lib/mime`

Changing
```
parts.map { |part| "\r\n" + part.to_s(opts) + "\r\n" }
```
to
```
parts.map { |part| "\r\n" + part.to_s(opts).force_encoding('UTF-8') + "\r\n" }
```
seems to have addressed the issue.

I'll attach a pull request to this issue tomorrow morning with my change.