Generate string representation of XML element.
All subelements are included. If encoding is "unicode", a string
is returned. Otherwise a bytestring is returned.
*element* is an Element instance, *encoding* is an optional output
encoding defaulting to US-ASCII, *method* is an optional output which
can be one of "xml" (default), "html", "text" or "c14n",
*default_namespace* sets the default XML namespace (for "xmlns").
Returns an (optionally) encoded string containing the XML data.