/*
 * call-seq:
 *   field.to_str                                     => "name=value"
 *   field.to_s                                       => "name=value"
 * 
 * Obtain a String representation of this PostField in url-encoded 
 * format. This is used to construct the post data for non-multipart
 * POSTs.
 * 
 * Only content fields may be converted to strings.
 */
static VALUE ruby_curl_postfield_to_str(VALUE self) {