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
following issue #6004 (fixed by PR #6006).
Added 2 dedicated tests.
Unfortunately I could not add a test for detectRetina option that easily, because since the move to Rollup, Browser values became internal variables, and overriding them later on does not have any effect on Leaflet internal references… Not sure how to workaround this for the sake of testing.
