Check that contextfunction, etc. is True

[davidism]

In order to determine if the context should be passed to a function or filter, Jinja has the contextfunction and contextfilter decorators which set corresponding attributes on the functions. However, they're checked with if getattr(f, "contextfunction", 0), which causes issues for objects that are callable and have permissive __getattr__ functions, such as Mock.

Changing everything to default to False and check is True, would be good for consistency.

The full list is contextfunction, evalcontextfunction, environmentfunction, contextfilter, evalcontextfilter, and environmentcontextfilter.

Extracted from #1139

[mitsuhiko]

I'm honestly not sure if it's a good idea to pass mock objects to a template in the first place. FWIW the reason 0 is used is because this code was faster on 2.x where the constant optimization was not applied yet.

[davidism]

I agree it's not, but there was another example of it coming up outside testing and it's a straightforward fix. 0 was only used in one place, False was used everywhere else already.