[auth] log and return 401 when checking mTLS for http writes with una…

…uthenticated reads enabled

Access was previously being denied correctly, but we were returning
success (200) to the client and not logging an event on the server.

server/http.go

@@ -264,6 +264,8 @@ func (h *httpCache) CacheHandler(w http.ResponseWriter, r *http.Request) {
 
 	case http.MethodPut:
 		if h.checkClientCertForWrites && !h.hasValidClientCert(w, r) {
+			http.Error(w, "Authentication required for write access", http.StatusUnauthorized)
+			h.logResponse(http.StatusUnauthorized, r)
 			return
 		}