rem: Ns_ConnSetHeaders(conn, "Last-Modified", Ns_HttpTime(&ds, mtime));
add: Ns_ConnCondSetHeaders(conn, "Last-Modified", Ns_HttpTime(&ds, mtime));
context: Ns_ConnSetLastModifiedHeader(Ns_Conn *conn, time_t *mtime){    Ns_DString ds;    Ns_DStringInit(&ds);    Ns_ConnSetHeaders(conn, "Last-Modified", Ns_HttpTime(&ds, mtime));    Ns_DStringFree(&ds);}