[1.6.x] Fixed #23900 -- Added missing imports in code example.
[1.6.x] Fixed #23900 -- Added missing imports in code example.

The Preventing header injection example included
classes that are not imported.

Thanks to Collin Anderson and Berker Peksağ for the
reviews.

Backport of 0d74209ef66111f29f122d5f5fdf8e2964890cc5 from master.

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