Unreleased Changes in The Mainline

Breaking Changes

A message with multipart/mixed as the root with multipart/related as a child part was not structurally parsed correctly, producing incorrect parts when using mimepart:get_simple_structure. Thanks to @kayozaki! #506
typing.lua: couldn't distinguish false from unset for a boolean field with default of true, such as those used in mail_auth.lua. Thanks to @kayozaki! #505