/*
 * call-seq:
 *   easy.on_debug { |type, data| ... }               => <old handler>
 *
 * Assign or remove the +on_debug+ handler for this Curl::Easy instance.
 * To remove a previously-supplied handler, call this method with no
 * attached block.
 *
 * The +on_debug+ handler, if configured, will receive detailed information
 * from libcurl during the perform call. This can be useful for debugging.
 * Setting a debug handler overrides libcurl's internal handler, disabling
 * any output from +verbose+, if set.
 *
 * The type argument will match one of the Curl::Easy::CURLINFO_XXXX
 * constants, and specifies the kind of information contained in the
 * data. The data is passed as a String.
 */
static VALUE ruby_curl_easy_on_debug_set(int argc, VALUE *argv, VALUE self) {