add None value for cache extension

[romanyakovlev]

As Django docs said The fragment is cached forever if timeout is None. However when I set timeout to None get this error: "cache" tag got a non-integer timeout value: None. So I made a little fix.

[wizpig64]

It looks like this change is much like the one in django 2.0 from four years ago, that allowed a None timeout:

• https://github.com/django/django/pull/8120
• https://code.djangoproject.com/ticket/27882

So this gets approval from me. I contributed a test and added a line in the changelog.

[wizpig64]

hmm, i thought that doing it that way would have merged [and closed] this PR as well, but maybe it didn't because the merge was done with a rebase. @auvipy could you merge or close this?

[auvipy]

the commit is merged