Fix improper header separator for X-OfflineIMAP header

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>

offlineimap/folder/GmailMaildir.py

@@ -141,7 +141,7 @@ class GmailMaildirFolder(MaildirFolder):
         # Change labels into content
         labels_str = imaputil.format_labels_string(self.labelsheader,
           sorted(labels | ignoredlabels))
-        content = self.addmessageheader(content, self.labelsheader, labels_str)
+        content = self.addmessageheader(content, '\n', self.labelsheader, labels_str)
         rtime = self.messagelist[uid].get('rtime', None)
 
         # write file with new labels to a unique file in tmp