PoorMansMailServer.ImapServer doesn't provide a proper IMAPMessage when HeaderMapper is provided for adapter

When we don't provide HeaderMapper to the ImapIdleChannelAdapter (for example), an internal transformation is applied to the MimeMessage - IntegrationMimeMessage.

With the HeaderMapper we end up with this results after target receiving:

payload = "To: [email protected]\r\nFrom: [email protected]\r\nSubject: Test Email\r\n\r\nfoo\r\n"
headers = {[email protected]}  size = 16
...
 1 = {Col[email protected]1949} "mail_to" -> 
  key = "mail_to"
  value = {String[1]@1969} 
   0 = "Bar <[email protected]>"
...
 5 = {Col[email protected]1953} "mail_from" -> "Foo <[email protected]>"
  key = "mail_from"
  value = "Foo <[email protected]>"

POP3 works well. That's why I assume that we have something wrong in the ImapServer.doRun().

The simplest way to reproduce is ImapMailReceiverTests.testIdleWithMessageMapping() and additional asserts in the code block:

else {
	org.springframework.messaging.Message<?> received = channel.receive(10000);
	assertNotNull(received);
	assertNotNull(received.getHeaders().get(MailHeaders.RAW_HEADERS));
	assertThat((String) received.getHeaders().get(MailHeaders.CONTENT_TYPE),
			equalTo("TEXT/PLAIN; charset=ISO-8859-1"));
	assertThat((String) received.getHeaders().get(MessageHeaders.CONTENT_TYPE),
			equalTo("TEXT/PLAIN; charset=ISO-8859-1"));
}

for expected FROM, TO and payload.