Account for message/global enclosed Content Type

[benjaminwhtan]

https://tools.ietf.org/html/rfc6532#section-3.7

This media type provides
functionality similar to the message/rfc822 content type for email
messages with internationalized email headers. When there is a
need to embed or return such content in another message, there is
generally an option to use this media type and leave the content
unchanged or down-convert the content to message/rfc822.

This will allow us to correctly parse content embedded within message/global Content Types. Currently, this will result in an exception with the message Malformed MIME message.

[mailgun-ci]

Can one of the admins verify this patch?

[thrawn01]

Thank you for making this change, We have not been as attentive to this repo or to internationalization support as we should! I'll approve this PR.