/*
 * call-seq:
 *   easy.on_body { |body_data| ... }                 => <old handler>
 *
 * Assign or remove the +on_body+ handler for this Curl::Easy instance.
 * To remove a previously-supplied handler, call this method with no
 * attached block.
 *
 * The +on_body+ handler is called for each chunk of response body passed back
 * by libcurl during +perform+. It should perform any processing necessary,
 * and return the actual number of bytes handled. Normally, this will
 * equal the length of the data string, and CURL will continue processing.
 * If the returned length does not equal the input length, CURL will abort
 * the processing with a Curl::Err::AbortedByCallbackError.
 */
static VALUE ruby_curl_easy_on_body_set(int argc, VALUE *argv, VALUE self) {