assert: handle (deep) equal(NaN, NaN) as being identical

This aligns the `equal` and `deepEqual()` implementations with the
strict versions by accepting `NaN` as being identical in case both
sides are NaN.

Refs: https://github.com/nodejs/node/issues/30350#issuecomment-552191641

PR-URL: https://github.com/nodejs/node/pull/30766


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>