mime_content_multipart can be indexed.
Add. Support MD5 library (http://www.keplerproject.org/md5).
Add. Test to CRAM-MD5 authentication (RFC 2195).
Fix. Test optional facility (e.g. pop3.auth_apop / charset encoding)
```lua
assert(msg.content.is_multi)
cs = msg.content[1]:charset()
```
Fix. Test on Windows