/*
 * call-seq:
 *   easy.on_header { |header_data| ... }             => <old handler>
 *
 * Assign or remove the +on_header+ handler for this Curl::Easy instance.
 * To remove a previously-supplied handler, call this method with no
 * attached block.
 *
 * The +on_header+ handler is called for each chunk of response header passed
 * back by libcurl during +perform+. The semantics are the same as for the
 * block supplied to +on_body+.
 */
static VALUE ruby_curl_easy_on_header_set(int argc, VALUE *argv, VALUE self) {