Placeholder is missing on Heading block

[marecar3]

In order to fix an issue #627 .
We have intentionally created an issue with missing placeholder on heading block.

This issue is created because we need the heading block to start with the <h> tag, but when that happens then Aztec loses its placeholder.

[hypest]

We will need to come up with a robust sollution for this one this time. Most probably a solution that will make Aztec more flexible on how it determines when/how to show the placeholder.

[koke]

Also we should prioritize this before block work, since I believe this will break in a similar way on any non-paragraph text block

[marecar3]

Fixed in this PR: #739