Made 'default' and 'default_if_none' filters bypass FastDev template exceptions

[Naggafin]

this change makes it so that non-existent context variables in templates will not raise a FastDev exception if they are being filtered by |default or |default_if_none. instead, it defaults to standard Django template behavior, returning None on the lookup, which gets passed to either of those filters. this is probably what most people would have expected as standard behavior from FastDev in templates–it certainly makes sense and is a succinct way to populate a dubious context variable that may or may not have been declared in the view context.

[boxed]

I like it! Could you add a trivial test for this?

[boxed]

I removed the behavior that |default_if_none does not crash on non-existent variables. The filter name explicitly says to check for None, so not-existing doesn't qualify. You would have to do non_existent|default:None|default_if_none:"foo" in that case I think.