The documentation promises that host validation is disabled when
DEBUG=True, that all hostnames are accepted. Domains not compliant with
RFC 1034/1035 were however being validated, this validation has now been
removed when DEBUG=True.
Additionally, when DEBUG=False a more detailed SuspiciousOperation
exception message is provided when host validation fails because the
hostname is not RFC 1034/1035 compliant.
SuspiciousOperations have been differentiated into subclasses, and
are now logged to a 'django.security.*' logger. SuspiciousOperations
that reach django.core.handlers.base.BaseHandler will now return a 400
instead of a 500.
Thanks to tiwoc for the report, and Carl Meyer and Donald Stufft
for review.