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).create(values)
        self._update_cron()
        return res
    @api.multi
    def write(self, values):
        res = super(FetchmailServer, self).write(values)
        self._update_cron()
        return res
    @api.multi
    def unlink(self):