prepare method
Prepare the object to be written to the stream
Implementation
@override
void prepare() {
super.prepare();
params[PdfNameTokens.type] = const PdfName(PdfNameTokens.metadata);
params[PdfNameTokens.subtype] = const PdfName(PdfNameTokens.xml);
params.data = Uint8List.fromList(utf8.encode(metadata.toString()));
}