Fix for Enter.key pressed at the end of Heading block that does create another Heading block

[daniloercoli]

This PR fixes the problem where pressing enter.KEY at the end of a Heading block, it does create another Heading block. Fixes #734

Expectation: Pressing enter on a Heading block, should create a Paragraph block. Same as on any other "split-able" block.

Testing A

• Start the demo app
• Put the caret at the end of a Heading block
• Press enter.key
• It should create a new para after it

Testing B

• Start the demo app
• Put the caret at the middle of a Heading block
• Press enter.key
• It should create a new heading after it, with the content on the right of the caret

This PR is a bug fix, but we may need to put an eye on this other PR here that modifies the default behaviour of the Heading block when pressing Enter.key at the beginning of it.

[mzorz]

LGTM