(For whoever reviews this)

I have mixed feelings about this. This is needed for when we use `@helpers.change_config` a few lines below. We're changing `ckan.auth.*` config variables. They're cached the first time someone calls `new_authz.check_config_permission()`, so we need to clear the cache to allow our changes to work.

My doubt is: is it better to make the user of `@helpers.change_config` call this if she's messing with `ckan.auth.*`, or should we call it on `@helpers.change_config` itself? The drawback of doing it is, apart that we might surprise the user (e.g. wtf? `change_config` is reseting the auth cache?!), we might break some test that wants to use it but can't pass with this side effect.

The clear advantage is that it makes the decorator easier to use (in this case). If we clear the auth cache, we might end up clearing other cached values that we stumble upon.

What do you think?

/cc @seanh