/*
 * call-seq:
 *   easy.on_success { |easy| ... }                   => <old handler>
 *
 * Assign or remove the +on_success+ handler for this Curl::Easy instance.
 * To remove a previously-supplied handler, call this method with no
 * attached block.
 *
 * The +on_success+ handler is called when the request is finished with a
 * status of 20x
 */
static VALUE ruby_curl_easy_on_success_set(int argc, VALUE *argv, VALUE self) {