Generated HTML for edited links containing multibyte characters is weird.

This is another side-effect of the underlying bug behind #853. #854 improves how the editor behaves when tapping on the "link" button in the toolbar, but the output HTML stills isn't great.

Expected behavior

After creating a link containing multibyte characters and then editing it, there's only one <a> element in the HTML output:

<a href="https://example.com">❤️ test 👍</a>

Actual behavior

Multiple <a> elements are present in the output:

<p>
<a href="http://example.com">❤️</a><a href="http://example.com"> test </a><a href="http://example.com">👍</a>
</p>

Steps to reproduce the behavior

1. Insert a link with any URL, consisting of multi-byte characters (two emojis is probably the easiest).
2. Put a cursor between the two characters.
3. Start typing something, I usually add test in between the two.
4. Look at the resulting HTML.

Inizia riproducendo il problema in AztecEditor-iOS con il pulsante per i link della barra degli strumenti: crea un link attorno a caratteri multibyte, modificalo tra i caratteri e ispeziona l’HTML generato. La correzione è completa quando il testo modificato è contenuto in un unico elemento con l’URL prevista, invece che in più elementi.