[EMAIL-142] HtmlEmail with both an html and plain text content but no attachments or embedded images will now be created with mimetype multipart/alternative instead of multipart/mixed. Thanks to Marcin Tomiak.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/email/trunk@1629061 13f79535-47bb-0310-9956-ffa450edef68