rgw: Fixed Content-Type always needing to be in the signature

This fixes an issue where the content-type header, if present in the
request, is assumed to always be required in the signature for the
request even if content type is NOT listed in the `X-Amz-SignedHeaders`
for that request.

Bug report: https://tracker.ceph.com/issues/79674

Signed-off-by: Elliot Courant <me@elliotcourant.dev>