//try with string
propertyType.setValue("myvalue");
Encoder encoder = new Encoder(new WFSConfiguration());
//System.out.println(encoder.encodeAsString(update, WFS.Update));
Document doc = encoder.encodeAsDOM(update, WFS.Update);
XMLAssert.assertXpathEvaluatesTo("mn:mytypename", "//wfs:Update/@typeName", doc);
XMLAssert.assertXpathEvaluatesTo("mn:myproperty", "//wfs:Update/wfs:Property/wfs:Name", doc);
XMLAssert.assertXpathEvaluatesTo("myvalue", "//wfs:Update/wfs:Property/wfs:Value", doc);
XMLAssert.assertXpathEvaluatesTo("someid", "//wfs:Update/ogc:Filter/ogc:FeatureId/@fid", doc);