when passing `crossOrigin: ''` (empty string) option, the `crossorigin` attribute should also be set an empty string value (valid keyword, with same effect as `"anonymous"` value). Need to add this test, because in JavaScript the empty string is a falsy value that could have been caught as `false` and led to no `crossorigin` attribute set.
* Test(ImageOverlay): crossOrigin option to IMG crossorigin attribute
typical values: false (default), true => '', <string> => <string>
now that a string value can be passed to this option.
* Test(TileLayer): crossOrigin option to IMG crossorigin attribute
typical values: false (default), true => '', <string> => <string>
now that a string value can be passed to this option.
* Style(TileLayerSpec): remove commented code
* Style(TileLayerSpec): remove more commented code
