Data: Make the data object a regular object again

Data: Make the data object a regular object again

The change in gh-4603 made the object returned by `elem.data()`
a prototype-less object. That's a desired change to support keys
colliding with `Object.prototype` properties but it's also a breaking
change so it has to wait for jQuery 4.0.0.

A 3.x-only test was added to avoid breaking it in the future on this
branch.

Fixes gh-4665
Ref gh-4603
Closes gh-4666