* See <a href='http://tools.ietf.org/html/rfc1864'>RFC1864</a>.
* @return raw encoded content-md5
* or null if no header exists
*/
public String getContentMD5Raw() {
ContentMD5Field contentMD5Field = (ContentMD5Field) fields.get(CONTENT_MD5);
return contentMD5Field != null ? contentMD5Field.getMD5Raw() : null;
}