Follow-up to JDK-8269630.

At this point in time ClipboardContent API extends a HashMap, which exposes HashMap's put() method directly. This allows users to put() anything into ClipboardContent, which then can cause different behaviors - Windows will accept the object and return it since it cares less about the contents, while Linux requires some more work depending on data type. At the time of writing this I'm unsure about macOS yet, it would require checking.

We should establish how the API behaves across all JavaFX platforms and also decide what to do in more questionable situations, like the one mentioned in JDK-8269630.