For servers without UIDPLUS we are inserting additional header
just after transformation '\n' -> CRLF was done. addmessageheaders()
was written to work with just '\n' as the separator, so X-OfflineIMAP
header wasn't preceeded by the CRLF, but just by '\n'.
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>