/*
 * call-seq:
 *   field.set_content_proc { |field| ... }           => <old proc>
 * 
 * Set a content proc for this field. This proc will be called during the
 * perform to supply the content for this field, overriding any setting
 * of +content+ or +local_file+.
 */
static VALUE ruby_curl_postfield_content_proc_set(int argc, VALUE *argv, VALUE self) {