kio_pop3: Fix "missing mimetype" warnings.

The POP3 kioslave doesn't emit a mimetype before outputting date for the
LIST and UIDL POP3 commands, which leads to warnings from KIO about
missing mimetypes. Fixed by using a "text/plain" mimetype.

Likewise the RETR command fails to output a mimetype in the case where
the caller didn't provide a msg id to download (this should be an error
anyways, but currently we emit an empty message instead). This is fixed
by treating as an error, and a separate patch eliminates the
missing-msg-id cases I was seeing from Akonadi.

REVIEW:124987
(cherry picked from commit 544410c9)