Add new tests for FeatureOverlay

In the process of creating these tests, a bug with setting this._style
was also fixed.

src/ol/featureoverlay.js

@@ -65,7 +65,7 @@ ol.FeatureOverlay = function(opt_options) {
    * @private
    * @type {ol.style.Style|Array.<ol.style.Style>|ol.feature.StyleFunction}
    */
-  this.style_ = null;
+  this.style_ = goog.isDef(options.style) ? options.style : null;
 
   /**
    * @private

test/spec/ol/featureoverlay.test.js

@@ -0,0 +1,34 @@
+goog.provide('ol.test.FeatureOverlay');
+
+describe('ol.FeatureOverlay', function() {
+
+  describe('constructor', function() {
+
+    it('creates an new feature overlay', function() {
+      var featureOverlay = new ol.FeatureOverlay();
+      expect(featureOverlay).to.be.a(ol.FeatureOverlay);
+    });
+
+    it('takes features', function() {
+      var featureOverlay = new ol.FeatureOverlay({
+        features: [new ol.Feature(new ol.geom.Point([0, 0]))]
+      });
+      expect(featureOverlay.getFeatures().getLength()).to.be(1);
+    });
+
+    it('takes a style', function() {
+      var style = [new ol.style.Style()];
+      var featureOverlay = new ol.FeatureOverlay({
+        style: [new ol.style.Style()]
+      });
+      expect(featureOverlay.getStyle()).to.eql(style);
+      expect(featureOverlay.getStyleFunction()()).to.eql(style);
+    });
+
+  });
+});
+
+goog.require('ol.Feature');
+goog.require('ol.FeatureOverlay');
+goog.require('ol.geom.Point');
+goog.require('ol.style.Style');