Bugfix: Broken HTML after H1

[jleandroperez]

Scenario: Corrupt HTML

1. Type Heading
2. Select the new word and toggle H1
3. Press return
4. Type a two characters
5. Delete one of the two characters

Verify that the HTML generated looks as follows:
<h1>Heading<br></h1>A

Scenario: Randomness

1. Launch the empty editor
2. Toggle H1
3. Enter some text. Hit return
4. Enter some text

Verify that the second line is, effectively, in Paragraph style

Nukes Zero Width Characters Usage in HeaderFormatter (Ref. #414)
Closes #407
Closes #415

Needs Review: @diegoreymendez
cc @SergioEstevao

Thanks in advance gentlemen!

[diegoreymendez]

This is pure gold. Nice work @jleandroperez !

[jleandroperez]

Woooo thanks Diego!!!