use is_present_hf to check if Expires header is set

kamailio/registrar-role.cfg

@@ -136,7 +136,7 @@ route[ATTEMPT_AUTHORIZATION]
     if(@contact.expires) {
         $var(expires) = @contact.expires;
     } else { 
-        if($hdr(Expires)) {
+        if(is_present_hf("Expires")) {
            $var(expires) = $hdr(Expires);
         } else {
            $var(expires) = REGISTRAR_MIN_EXPIRES;