append(s, charset=None, errors='strict')
Append the string s to the MIME header.
Optional charset, if given, should be a Charset instance (see email.charset) or the name of a character set, which will be converted to a Charset instance. A value of None (the default) means that the charset given in the constructor is used.
s may be an instance of bytes or str. If it is an instance of bytes, then charset is the encoding of that byte string, and a UnicodeError will be raised if the string canno