[FIX] decode_header is broken in Python 2.

The function email.header.decode_header may corrupt some header.  For instance, the Subject "Courriel reçu" lose the white space and is saved as "Courrielreçu".

This is due to issue http://bugs.python.org/issue1079, solved in Python 3.3.  

The package future has a backport.  So using it solves this issue.