MDL-48593 messageinbound: Correct header retrieval from IMAP server

Header retrieval should only be performed on the base message, and not each
part of the message as is currently performed.

This came to light because Google changed their IMAP server such that, if
you inform the data query that you wish to retrieve the headerText for a
part which does not contain a MIME header, then no message is retrieved in
the search causing a general failure of the IMAP fetch.

In normal circumstances, headers should only be a part of the base message
part.