Fix method HttpHeadersNonValidated uses to get HeaderDescriptor (#62150)

Fix method HttpHeadersNonValidated uses to get HeaderDescriptor (#62150)

We have two non-public methods for getting a HeaderDescriptor from a string name, one static on HeaderDescriptor and one instance on HttpHeaders.  HttpHeadersNonValidated was using the former, but this then doesn't take into account custom logic used by HttpHeaders in other methods, like TryAddWithoutValidation, which means some methods on HttpHeadersNonValidated can't find the corresponding headers.