Adds srsDimension to the pos and posList nodes when writing a WFS transaction

The WFS transaction failed if the srsDimension was not present on the
geometries.
Unit tests have been changed to accomodate the srsDimension attribute.

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

@@ -524,7 +524,7 @@ describe('ol.format.WFS', function() {
           '      <gml:Polygon xmlns:gml="http://www.opengis.net/gml">' +
           '        <gml:exterior>' +
           '          <gml:LinearRing>' +
-          '            <gml:posList>' +
+          '            <gml:posList srsDimension="2">' +
           '              10 20 10 25 15 25 15 20 10 20' +
           '            </gml:posList>' +
           '          </gml:LinearRing>' +
@@ -561,7 +561,7 @@ describe('ol.format.WFS', function() {
           '      <gml:Polygon xmlns:gml="http://www.opengis.net/gml">' +
           '        <gml:exterior>' +
           '          <gml:LinearRing>' +
-          '            <gml:posList>' +
+          '            <gml:posList srsDimension="2">' +
           '              10 20 10 25 15 25 15 20 10 20' +
           '            </gml:posList>' +
           '          </gml:LinearRing>' +