Caching something that shouldn't be cached is a potential source of
bugs and security vulnerabilities. For example, one could write a
form helper that outputs a request-specific auth token, only for
the helper to be used inside of a `cache` block.

In the GitHub application, we implemented a caching? method and used
it to raise an error if a specific code path is being cached that
we don't want to be cached.

I've credited its original author, @btoews.

Co-authored-by: Ben Toews <mastahyeti@gmail.com>
Co-authored-by: John Hawthorn <jhawthorn@github.com>
Co-authored-by: Kasper Timm Hansen <kaspth@gmail.com>