XmlTokenSource.save() does not use specified character encoding

Saving the in-memory XML via the .save(Writer w, XmlOptions options) method
inherited from XmlTokenSource does not honour the specified character encoding.

Example:

{
// ... code above

// GENERATE THE RESPONSE OBJECT.
XLSType responseXLS = processXLSDocument(requestXLS,
globalRequestContext);

// SERIALIZE THE RESPONSE OBJECT.
ByteArrayOutputStream bos = new ByteArrayOutputStream();
OutputStreamWriter osw = new OutputStreamWriter(bos, "ISO-8859-1");

// Serialize the response.
xlsDoc = XLSDocument.Factory.newInstance();
xlsDoc.setXLS(responseXLS);

opts = new XmlOptions();
opts.setCharacterEncoding("ISO-8859-1");
opts.setUseDefaultNamespace();
opts.setSaveAggresiveNamespaces();

xlsDoc.save(osw, opts);
osw.flush();
osw.close();

// ... code below

Always produces a document that is encoded in UTF-8 (xml header <?xml
version="1.0" encoding="UTF-8"?>).

Incidentally, I haven't been able to find the list of character encodings that
is supported by XMLBeans.