/*
 * call-seq:
 *   easy.header_str                                  => "response header"
 *
 * Return the response header from the previous call to +perform+. This
 * is populated by the default +on_header+ handler - if you supply
 * your own header handler, this string will be empty.
 */
static VALUE ruby_curl_easy_header_str_get(VALUE self) {