Make writeFilter version argument optional

* Add test for optional version argument to writeFilter
* Default to WFS 1.1.0 if no version passed to writeFilter

src/ol/format/WFS.js

@@ -1323,11 +1323,12 @@ function writeTimeInstant(node, time) {
  * Encode filter as WFS `Filter` and return the Node.
  *
  * @param {import("./filter/Filter.js").default} filter Filter.
- * @param {string} version Version.
+ * @param {string} opt_version WFS version. If not provided defaults to '1.1.0'
  * @return {Node} Result.
  * @api
  */
-export function writeFilter(filter, version) {
+export function writeFilter(filter, opt_version) {
+  const version = opt_version || '1.1.0';
   const child = createElementNS(getFilterNS(version), 'Filter');
   const context = {
     node: child,

test/spec/ol/format/wfs.test.js

@@ -1493,6 +1493,15 @@ describe('ol.format.WFS', function () {
       );
       expect(serialized).to.xmleql(parse(wfs1Filter));
     });
+    it('defaults to creating a WFS 1.x.x filter if no version specified', function () {
+      const serialized = writeFilter(
+        andFilter(
+          likeFilter('name', 'Mississippi*'),
+          equalToFilter('waterway', 'riverbank')
+        )
+      );
+      expect(serialized).to.xmleql(parse(wfs1Filter));
+    });
     it('creates a WFS 2.x.x filter', function () {
       const serialized = writeFilter(
         andFilter(