Fix WFS 2.0.0 writeFilter

* Add a test for WFS 2.0.0 filter encoding
* Use getFilterNS to ensure correct NS for filter child node

src/ol/format/WFS.js

@@ -1328,7 +1328,7 @@ function writeTimeInstant(node, time) {
  * @api
  */
 export function writeFilter(filter, version) {
-  const child = createElementNS(OGCNS[version], 'Filter');
+  const child = createElementNS(getFilterNS(version), 'Filter');
   const context = {
     node: child,
   };