We cannot cache keys because arrays are mutable. We rather want to cache
the arrays. This behaviour is tailor-made for the usage pattern strongs
params is designed for.

In a forthcoming commit I am going to add a test that covers why we need
to cache by value.

Every strong params instance has a live span of a request, the cache goes
away with the object. Since strong params have such a concrete intention,
it would be interesting to see if there are actually any real-world use
cases that are an actual leak, one that practically may matter.

I am not convinced that the theoretical leak has any practical consequences,
but if it can be shown there are, then I believe we should either get rid of
the cache (which is an optimization), or else wipe it in the mutating API.

This reverts commit e63be276.