[#59][HOTFIX] Headers multiline error

Closes #59 (closed)

• Updated vendor/django_mail/message.py to specify maxlen=0 (no max) on each Header instanciation so that long headers are not splitted into multi lines.

Fixed the current multiline subject issue, but not the SMIME issue (currently fixed by an ugly replace in prepareSignedMessage. smail module also has calls to headers, and could be copied locally for the same fix to be applied.