net/mail: use base64 encoding when needed in Address.String()
When the name of an Address contains non-ASCII characters,
Address.String() used mime.QEncoding to encode the name.
However certain characters are forbidden when an encoded-word is
in a phrase context (see RFC 2047 section 5.3) and these
characters are not encoded by mime.QEncoding.
In this case we now use mime.BEncoding (base64 encoding) so that
forbidden characters are also encoded.