Allow HTML to render when no filter_class is defined.

[ericholscher]

Previously it required a filter_class,
or else it would error when calling cls().
This now sets the filter context to None
if one does not exist.

This fixes #3559

[ericholscher]

Not sure what the best way to test this is, happy to add tests if someone points the way.

[jpadilla]

As a general note/nitpick can we go ahead and also make this look like the filter_query() method on top, basically just renaming cls for filter_class.

[jpadilla]

@ericholscher thanks for putting this together, looks good to me. We were already doing a similar check on the filter_queryset() method on top of that one. HTML rendering for filters has no tests that I know of yet, so I wouldn't really worry about it.

[jpadilla]

@ericholscher also I'm wondering what does that rendered HTML looks like with no filter since both filter templates use filter.form from context.

[lovelydinosaur]

happy to add tests if someone points the way.

An example, along with screenshots demonstrating the behavior in the None case would prob be first point of call. We could figure out a sensible test case following on from that.

[lovelydinosaur]

Gonna accept this for now, given bug fix nature of the 3.3.1 release.
Thanks @ericholscher!