implementation here just
 calls the <code>updateHeaders</code> method on each of its
 children BodyParts. <p>

 Note that the boundary parameter is already set up when
 a new and empty MimeMultipart object is created. <p>

 This method is called when the <code>saveChanges</code>
 method is invoked on the Message object containing this
 Multipart. This is typically done as part of the Message
 send process, however note that a client is free to call
 it any number of times. So if the header updating process 