* Fix case-insensitive pre-auth header check in doPing
HTTP headers are case-insensitive per RFC 7230, but the pre-auth
rejection check accessed x-reject-reason using an exact lowercase key.
Servers/proxies may return varying cases (e.g. X-Reject-Reason). Add a
getResponseHeader utility for case-insensitive lookup and use it in both
pre-auth check paths.
* Address PR feedback: optimize getResponseHeader and fix test description
Try exact match and lowercase match before falling back to a full scan
to avoid array allocation on every call. Fix test description to say
"uppercase" instead of "mixed case" to match the test data.
