[1.6.x] Fixed docs for EmailMessage extra_headers attribute
[1.6.x] Fixed docs for EmailMessage extra_headers attribute

The EmailMessage documentation describes the initialization parameters
and states they 'can be set at any time prior to calling the send()
method.'. However, the 'headers' parameter's corresponding attribute is
called 'extra_headers'.

Backport of cab72a439b from master

django-nonrel/django,dex4er/django,felixjimenez/django,django-nonrel/django,redhat-openstack/django,felixjimenez/django,felixjimenez/django,felixjimenez/django,django-nonrel/django,django-nonrel/django,redhat-openstack/django,redhat-openstack/django,dex4er/django,redhat-openstack/django,dex4er/django