Add a getUrls Method to ol.source.XYZ

Added the getUrls method to get access to the url / urls property.
When using a tileUrlFunction, getUrls should return null.
This should fix issue #3886

test/spec/ol/source/xyzsource.test.js

@@ -115,6 +115,61 @@ describe('ol.source.XYZ', function() {
 
   });
 
+  describe('#getUrls', function() {
+
+    var sourceOptions;
+    var source;
+    var url = 'http://geo.nls.uk/maps/towns/glasgow1857/{z}/{x}/{-y}.png';
+
+    beforeEach(function() {
+      sourceOptions = {
+        projection: 'EPSG:4326'
+      };
+    });
+
+    describe('using a "url" option', function() {
+      beforeEach(function() {
+        sourceOptions.url = url;
+        source = new ol.source.XYZ(sourceOptions);
+      });
+
+      it('returns the XYZ URL', function() {
+        var urls = source.getUrls();
+        expect(urls).to.be.eql([url]);
+      });
+
+    });
+
+    describe('using a "urls" option', function() {
+      beforeEach(function() {
+        sourceOptions.urls = ['some_xyz_url1', 'some_xyz_url2'];
+        source = new ol.source.XYZ(sourceOptions);
+      });
+
+      it('returns the XYZ URLs', function() {
+        var urls = source.getUrls();
+        expect(urls).to.be.eql(['some_xyz_url1', 'some_xyz_url2']);
+      });
+
+    });
+
+    describe('using a "tileUrlFunction"', function() {
+      beforeEach(function() {
+        sourceOptions.tileUrlFunction = function() {
+          return 'some_xyz_url';
+        };
+        source = new ol.source.XYZ(sourceOptions);
+      });
+
+      it('returns null', function() {
+        var urls = source.getUrls();
+        expect(urls).to.be(null);
+      });
+
+    });
+
+  });
+
 });
 
 goog.require('ol.TileCoord');