Describe the bug
Undoing a paragraph merge leaves behind a copy of the START_OF_SELECTED_AREA (0x0086) char.

To reproduce
Steps to reproduce the behavior:

1. Start off with this html:

<p>Word</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Press</p>
<!-- /wp:paragraph -->

2. Merge the “Press” block to the one above it
3. Undo
4. Inspect the html via the browser dev tools and verify that the special char is there before the Press word in the second block

Expected behavior
No copies of the special char should be left in the text/markup.

Screenshots

Desktop (please complete the following information):

• OS: macOS 10.14.4
• Browser Firefox 66.0.5
• Version on hash eea04cf