Make `stringify_keys` functions from MapUtils private

[begedin]

Problem

MapUtils.keys_to_string calls stringify_keys internally, to "deep-convert" all map keys into strings. stringify_keys should be private, but has been merged in as public.

[arubis]

This looks really straightforward, and so is a good way to run through the motions of a first contribution here :) Spec is green on my end; let me know if I'm overstepping at all by adding the spec for keys_to_string.