fix(js): support updating Element options

[salomvary]

I've not opened a separate Issue for this because showcasing the problem seemed easier in the form of a failing test case. I hope that's OK.

Before this fix, a call to update() like this:

update({panelContainer: document.createElement('div')})

would throw the following error:

TypeError: panelContainer.contains is not a function

The reason lies in an incorrect object deep merge implementation where
object entry values of Element type are merged like plain objects
instead of being replaced, resulting in broken non-Element entries.

This change fixes the issue with HTML Elements (Nodes of any kind to be
more precise) but it might be worth considering replacing the current
mergeDeep implementation with something more robust and future proof.
Deep merging of objects surprisingly hard to get right, see the
implementation in Lodash for example:
https://github.com/lodash/lodash/blob/4.17.15/lodash.js#L3633

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[francoischalifour]

Great—thank you very much for the contribution!

[Haroenv]

thanks!