Use NullStore as the default Digestor.cache if ActionView::Base.cache_template_loading is disabled

This fixes the issues discussed in [rails/cache_digests#48](https://github.com/rails/cache_digests/pull/48) and allows cache digests to be recalculated when templates are changed in the development environment (or any environment with the `cache_template_loading` config disabled) without requiring an application restart.

Note that this applies to `4-0-stable` and `4-0-0` as well.