code: public Encoding getEncoding() {
  		String value = first(ENCODING);
  		return (value == null) ? null : Encoding.get(value);
  	}
docs: <p>
  Gets the ENCODING parameter value.
  </p>
  <p>
  This parameter defines how the property value is encoded (for example,
  "base64" for a binary value).
  </p>
  @return the encoding or null if not set
  @see <a href="http://tools.ietf.org/html/rfc5545#page-18">RFC 5545
  p.18-9</a>