Encodes a string containing non-US-ASCII characters according to RFC-1522, returns
the resultant encoded string.
Use this method to encode a non-US-ASCII subject, sender's name or recipient's name, e.g.
Mail.Subject = Mail.EncodeHeader("La fête à Jean-Sébastien Bach")
or
Mail.AddAddress "oystein@somecompany.no", Mail.EncodeHeader("Øystein")
Charset can be set to "UTF-8" or any valid character set code
described in Chapter 6.