* See <a href='http://tools.ietf.org/html/rfc2557'>RFC2557</a>
* @return the URL content-location
* or null if no header exists
*/
public String getContentLocation() {
ContentLocationField contentLocationField =
(ContentLocationField) fields.get(CONTENT_LOCATION);
return contentLocationField != null ? contentLocationField.getLocation() : null;
}