Check for DPoPNonce with case-insensitive TryGetValues method

[drasticactions]

When checking for "DPoP-Nonce," this method checks against the Key value in the headers and uses a string comparison. This can fail because that header, like any other, can be case-insensitive, leading to issues where it can't get the value properly even though it is in the response.

TryGetValues will search for headers regardless of the case. Switching to that here, this method will work in all cases (pardon the pun, it took me hours of debugging to figure out this is what was broken, lol).