Fix: If opening <h[1-6]> is at the end of a line, merge it with the next line

[VolkerChristian]

Sometimes the opening <h[1-6]> is at the end of line e.g.

<h1 class="heading-element">  
<code>foo1</code></h1>

This shall became

<h1 class="heading-element"><code>foo1</code></h1>

This commit fixes this issue.