http: simplify the check for auth methods

http: simplify the check for auth methods

Avoids having to use the correct index into the line. Avoids repeated
use of is_valid_auth_separator.

Require that the following letter is not an alnum instead of checking
explicitly for ch == '\0' || ch == ',' || ISSPACE(ch). After all, the
point is to not erroneously match another auth string using the same
prefix.

Follow-up to b75620b9a05c0f0d03bd

Closes #16406