@Override
    public void startDocument() throws SAXException {
        if (handler != null) {
            handler.startDocument();
        } else {
            super.startDocument();
        }
    }
Buffers a startDocument event if buffer is set to true.

@throws SAXException Not thrown.
@see stroom.pipeline.filter.AbstractXMLFilter#startDocument()
stroom-pipeline/src/main/java/stroom/pipeline/filter/BufferFilter.java