Fixes #3867

Description

• Hitting Enter in a Heading throws an TypeError (described in Heading: Uncaught TypeError when pressing Enter #3867) due to an onKeyDown event propagating when splitting content, as @ephox-mogran noticed.
• While fixing and testing the above, I noticed errors thrown when merging lists into paragraphs, though the same issue happened when transforming lists into paragraphs and quotes.

How Has This Been Tested?

1. Start an empty post.
2. Add a heading with text.
3. With the cursor at the end of the heading's text field, paste a paragraph of HTML grabbed from somewhere—see the attached example after this list.
4. Ensure no errors are logged and that the text is pasted into a new paragraph, retaining the original heading as is.
5. After the paragraph, add a list with more than one item.
6. With the cursor at the beginning of the list, hit backspace.
7. Ensure no errors are logged and that the list is correctly transformed into paragraphs, the first of which merged into the original paragraph.

Attachment 1: Example HTML paragraph

Letterpress printing is a technique of relief printing using a printing press, a process by which many copies are produced by repeated direct impression of an inked, raised surface against sheets or a continuous roll of paper. A worker composes and locks movable type into the "bed" or "chase" of a press, inks it, and presses paper against it to transfer the ink from the type which creates an impression on the paper.

Screenshots (jpeg or gifs if applicable):

Before	After

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.