Problem with Stax2ByteArraySource, encodings other than UTF-8

[cowtowncoder]

(note: created for FasterXML/jackson-dataformat-xml#428)

Looks like construction of an XMLStreamReader with code like:

XMLStreamReader getReader(byte[] b) throws XMLStreamException
{
    XMLInputFactory2 f = ...
    Stax2ByteArraySource src = new Stax2ByteArraySource(b, 0, b.length);
    return f.createXMLStreamReader(src);
}

fails with an NPE.