[FIX] mail: missing spaces in email subjects composed of several encoding

sometimes some blank spaces are lost in subject of some incoming messages

There is a bug in `decode_header` in Python < 3.3,
which leads to lost some spaces in the email subjects
when several encodings are used in this subject.

See
- Issue: http://bugs.python.org/issue1079
- Fix: https://hg.python.org/cpython/rev/8c03fe231877

Joining the strings returned in the result of `decode_header`
solves most cases. Only extreme cases, like having
a subject with several different encodings following
each other without white spaces between them could lead
to have extra spaces in the subject. It won't happen
most of the time.

Closes #6629

addons/mail/mail_message.py

@@ -41,6 +41,8 @@ def decode(text):
     """Returns unicode() string conversion of the the given encoded smtp header text"""
     if text:
         text = decode_header(text.replace('\r', ''))
+        # The joining space will not be needed as of Python 3.3
+        # See https://hg.python.org/cpython/rev/8c03fe231877
         return ' '.join([tools.ustr(x[0], x[1]) for x in text])