HeaderFormatter: Back Delete should merge Header Styles

[jleandroperez]

1. Enter two paragraphs (h1 + h2):

<h1>Header 1 </h1>
<h2>Header 2 </h2>

2. Back delete just before Header 2

As a result, the text font on Header 2 should switch to Header 1 font size. Aztec currently ends up rendering a single paragraph but with both fonts size applied.

Props to @SergioEstevao for spotting this!

[diegoreymendez]

@jleandroperez - Some extra information for clarity: the NSParagraphStyle string attribute is fixed when fixParagraphStyleAttribute() is executed.

I think we need to start by checking if that method is being executed at all.