
 and MimeBody use this class for holding their headers.

 <hr> <strong>A note on RFC822 and MIME headers</strong><p>

 RFC822 and MIME header fields <strong>must</strong> contain only
 US-ASCII characters. If a header contains non US-ASCII characters,
 it must be encoded as per the rules in RFC 2047. The MimeUtility
 class provided in this package can be used to to achieve this.
 Callers of the <code>setHeader</code>, <code>addHeader</code>, and
 <code>addHeaderLine</code> methods are responsible for enfor