Case-insensitive Replay-Nonce headers

[adaugherity]

Split off from #859; I believe this is a separate bug which still exists:

Another potential bug is the checking of the Replay-Nonce header. I noticed in the certbot debug log (which was successful) that Sectigo was sending a lowercase replay-nonce header.

It appears that further down it does use grep -i, so in effect it's just causing an initial "bad nonce" and will succeed on retry, but we may as well fix the first attempt.

[adaugherity]

I can confirm that running this branch with the config in #883 does indeed eliminate the first badNonce response; it goes straight to externalAccountRequired.