force convert to utf-8

When I come to handle a xml with Chinese characters in it, I came to this lib. But soon the xml.etree.ElementTree prompt me that there's an error while parsing. Soon I discovered that the Chinese character is encoding with Unicode while the header of xml tells 'UTF-8'. So I changed the last line of dicttoxml.py with `.encode('utf-8')` added and which avoids the interpreting problem of Chinese character.