Impacted versions: 11.0

Steps to reproduce: fetchmal fails consistently while downloading messages

Current behavior: TypeError

Proposed patch for file addons/fetchmail/models/fetchmail.py:
```
--- fetchmail.py.orig	2017-10-30 20:53:32.590000000 +0000
+++ fetchmail.py	2017-10-30 20:53:53.890000000 +0000
@@ -204,7 +204,7 @@
                         pop_server.list()
                         for num in range(1, min(MAX_POP_MESSAGES, num_messages) + 1):
                             (header, messages, octets) = pop_server.retr(num)
-                            message = '\n'.join(messages)
+                            message = b'\n'.join(messages)
                             res_id = None
                             try:
                                 res_id = MailThread.with_context(**additionnal_context).message_process(server.object_id.model, message, save_original=server.original, strip_attachments=(not server.attach))
```



addons/fetchmail/models/fetchmail.py
import logging
import poplib
from imaplib import IMAP4, IMAP4_SSL
from poplib import POP3, POP3_SSL
from odoo import api, fields, models, tools, _
from odoo.exceptions import UserError
_logger = logging.getLogger(__name__)
MAX_POP_MESSAGES = 50
MAIL_TIMEOUT = 60
poplib._MAXLINE = 65536
class FetchmailServer(models.Model):