[1.7.x] Fixed #23815 -- Prevented UnicodeDecodeError in CSRF middleware

Thanks codeitloadit for the report, living180 for investigations
and Tim Graham for the review.
Backport of 27dd7e7271 from master.

docs/releases/1.7.3.txt

@@ -17,3 +17,6 @@ Bugfixes
   affect users who have subclassed
   ``django.contrib.auth.hashers.PBKDF2PasswordHasher`` to change the
   default value.
+
+* Fixed a crash in the CSRF middleware when handling non-ASCII referer header
+  (:ticket:`23815`).