Tests for get/setStyleFunction

It would be nice to also test the following:

    it('does not return user set property with the same name', function() {
      var feature = new ol.Feature({
        whatever: 'some value',
        styleFunction: 'another value'
      });
      expect(feature.getStyleFunction()).to.be(undefined);
    });

Unfortunately, in uncompiled code (or if we export `setStyleFunction`) this does not work.  Same goes for user set `id` properties (this will set our internal `id_` property).