Use drupal_convert_to_utf8() for UTF8 conversion

I've seen you fixed a core bug #371495: Error message from drupal_http_request() not UTF8 encoded I found a long time ago, but never solved in core:

    // Convert error message to utf-8.
    $errstr = iconv('utf-8', 'utf-8//IGNORE', $errstr);

After more investigation I guess you should better use http://api.drupal.org/api/drupal/includes--unicode.inc/function/drupal_c... . This function shows that iconv() may not available and than it fallback to others.

Until now I'm using this logic in linkchecker:

  // @todo drupal_http_request() may not provide an UTF8 encoded error message
  // what results in a database UPDATE failure. For more information, see
  // http://drupal.org/node/371495.
  // Workaround: ISO-8859-1 as source encoding may be wrong, but WFM.
  if (!empty($response->error) && !drupal_validate_utf8($response->error)) {
    $response->error = drupal_convert_to_utf8($response->error, 'ISO-8859-1');
  }