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
res = super(FetchmailServer, self).unlink()
        self._update_cron()
        return res
    @api.multi
    def set_draft(self):
        self.write({'state': 'draft'})
        return True
    @api.multi
    def connect(self):
        self.ensure_one()
        if self.type == 'imap':
            if self.is_ssl:
                connection = IMAP4_SSL(self.server, int(self.port))
            else:
                connection = IMAP4(self.server, int(self.port))
            connection.login(self.user, self.password)
        elif self.type == 'pop':
            if self.is_ssl:
                connection = POP3_SSL(self.server, int(self.port))
            else:
                connection = POP3(self.server, int(self.port))
            
            
            connection.user(self.user)
            connection.pass_(self.password)
        
        connection.sock.settimeout(MAIL_TIMEOUT)
        return connection
    @api.multi
    def button_confirm_login(self):