 Unicode string is examined for non US-ASCII
 characters. If the string contains only US-ASCII characters,
 it is returned as-is.  If the string contains non US-ASCII
 characters, it is first character-encoded using the specified
 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.</div>
<dl>
<dt><span class="p