Allow to use Saxon for the md2tei transformation.

In Java 8, the built-in XSLTC produces invalid XML (containing a
namespace binding to the prefix `xmlns`, cf. #16730). Saxon however
cannot write to a StAXResult, so we need an adapter to SAXResult.

Fixes #16730.

src/main/java/info/textgrid/services/aggregator/teicorpus/TEICorpusSerializer.java

@@ -33,9 +33,10 @@
 import javax.xml.stream.XMLOutputFactory;
 import javax.xml.stream.XMLStreamException;
 import javax.xml.stream.events.XMLEvent;
-import javax.xml.transform.stax.StAXResult;
+import javax.xml.transform.sax.SAXResult;
 import javax.xml.ws.Holder;
 
+import org.springframework.util.xml.StaxUtils;
 import com.google.common.base.Joiner;
 import com.google.common.base.Throwables;
 import com.google.common.collect.ImmutableList;
@@ -168,7 +169,7 @@ protected void walkAggregation(final ObjectType object, final boolean again) {
 		}
 
 		// writeHeader(object);
-		stack.writeHeader(new StAXResult(new XMLEventFilter(writer) {
+		XMLEventFilter xmlEventFilter = new XMLEventFilter(writer) {
 
 			@Override
 			public void add(final XMLEvent event) throws XMLStreamException {
@@ -176,8 +177,8 @@ public void add(final XMLEvent event) throws XMLStreamException {
 					super.add(event);
 			}
 
-		}
-		));
+		};
+		stack.writeHeader(new SAXResult(StaxUtils.createContentHandler(xmlEventFilter)));
 		}
 
 		super.walkAggregation(object, again);

src/main/java/info/textgrid/services/aggregator/teicorpus/TEIHeaderStack.java

@@ -156,7 +156,7 @@ protected Templates getStylesheet()
 					.getResourceAsStream("md2tei.xsl");
 			final StreamSource xslSource = new StreamSource(xslStream);
 
-			final TransformerFactory factory = TransformerFactory.newInstance("com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl", getClass().getClassLoader());
+			final TransformerFactory factory = TransformerFactory.newInstance();
 			stylesheet = factory.newTemplates(xslSource);
 		}
 		return stylesheet;