is returned as-is.  If the string contains non US-ASCII
 characters, it is first character-encoded using the platform's
 default charset, then transfer-encoded using either the B or 
 Q encoding. The resulting bytes are then returned as a Unicode 
 string containing only ASCII  characters. <p>

 Note that this method should be used to encode only 
 "unstructured" RFC 822 headers. <p>

 Example of usage:
 <blockquote><pre>

  MimePart part = ...
  String rawvalue = "FooBar Mailer, Japanese version 1.1"
  try