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.

Start by reproducing the issue in AztecEditor-iOS with the toolbar link button: create a link around multibyte characters, edit it between the characters, and inspect the generated HTML. The fix is complete when the edited text is contained in one element with the expected URL, rather than multiple elements.