JIRA: https://jira.spring.io/browse/INT-3994
If a header mapper is injected into the mail receiver, a mapped
message is result instead of a message with the raw `MimeMessage`.
`MimeMessage` properties are mapped as discrete headers; in addition
the raw email headers are provided as a multivalue map in the headers.
Handle Multipart
Since a Multipart content has a reference to the original MimeMessage,
convert to a simple byte[] by default.
Add an option in case the user wants to map the message but still
interpret the Multipart intact.
Namespace and Docs
Polishing
Polishing - PR Comments
439559f255