added TestCase
change headers default value
change decode method
edit test
add headers dlm bentuk dictionary
parameter headers sebelom priority
Changed the field into JSONField
Refactored the logic to add headers
Make headers not required by default, default to None
Not hard require django jsonfield
Merge branch 'jessica-master' of git://github.com/SeiryuZ/django-post_office

Conflicts:
	post_office/mail.py
	post_office/tests/models.py
Updated README.rst.