[Tooltip] - PopperProps broken starting from 4.9.5

[ValentinH]

Because of #19851, we are not able to create our own custom arrow for a Tooltip.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Passing an arrow modifier to PopperProps, throws Failed to execute 'contains' on 'Node': parameter 1 is not of type 'Node'.

Expected Behavior 🤔

The arrow is displayed properly.

Steps to Reproduce 🕹

Working version (4.9.4): https://codesandbox.io/s/material-demo-ltrt5
Broken version (4.9.5): https://codesandbox.io/s/material-demo-ddi25

Root cause

This is because deepmerge() replaces DOM nodes by {} objects.

Here a screenshot of the following console.log:

Your Environment 🌎

Tech	Version
Material-UI	v4.9.5
React	v16.13.0
Browser	Brave Version 1.3.118 Chromium: 80.0.3987.116 (Official Build) (64-bit)

[eps1lon]

It looks like it's actually attempting to merge the Elements. We probably just want to merge plain objects.

[ValentinH]

Should I create a PR to prevent deepMerge from merging Elements?