Make this a non-breaking change

src/ol/format/KML.js

@@ -1800,7 +1800,10 @@ function dataParser(node, objectStack) {
   if (name && featureObject.displayName) {
     featureObject[name] = {
       value: featureObject.value,
-      displayName: featureObject.displayName
+      displayName: featureObject.displayName,
+      toString: function() {
+        return featureObject.value;
+      }
     };
   } else if (name !== null) {
     featureObject[name] = featureObject.value;

test/spec/ol/format/kml.test.js

@@ -1670,6 +1670,25 @@ describe('ol.format.KML', function() {
           expect(f.get('bar')).to.be(undefined);
         });
 
+        it('can read ExtendedData with displayName instead of name', function() {
+          const text =
+              '<kml xmlns="http://earth.google.com/kml/2.2">' +
+              '  <Placemark xmlns="http://earth.google.com/kml/2.2">' +
+              '    <ExtendedData>' +
+              '      <Data>' +
+              '        <displayName>foo</displayName>' +
+              '        <value>bar</value>' +
+              '      </Data>' +
+              '    </ExtendedData>' +
+              '  </Placemark>' +
+              '</kml>';
+          const fs = format.readFeatures(text);
+          expect(fs).to.have.length(1);
+          const f = fs[0];
+          expect(f).to.be.an(Feature);
+          expect(f.get('foo')).to.be('bar');
+        });
+
         it('can read SchemaData', function() {
           const text =
               '<kml xmlns="http://earth.google.com/kml/2.2">' +
@@ -1711,10 +1730,8 @@ describe('ol.format.KML', function() {
           const f = fs[0];
           expect(f).to.be.an(Feature);
           expect(f.get('capital')).to.be('London');
-          expect(f.get('country')).to.eql({
-            value: 'United-Kingdom',
-            displayName: 'Country'
-          });
+          expect(f.get('country').value).to.be('United-Kingdom');
+          expect(f.get('country').displayName).to.be('Country');
         });
       });