CustomClassOption is not added when anchoring complex selected styled text

[anna-git]

Description

If you have a simple text that you wanna anchor, it works.
If you have a complex text, it's another story. Let's say something like :
<b>this is a</b><i><b> complicated</b> <strike><b>markup</b></strike></i>
If you have specified a button class to then function createLink, it won't be taken into account because it's going to handle the wrong node. The problem is the addClassToAnchors function, which won't traverse up to the parents node :

if (el.nodeName.toLowerCase() === 'a') {
                for (j = 0; j < classes.length; j += 1) {
                    el.classList.add(classes[j]);
                }
            } else {
                **el = el.getElementsByTagName('a');**
                for (i = 0; i < el.length; i += 1) {
                    for (j = 0; j < classes.length; j += 1) {
                        el[i].classList.add(classes[j]);
                    }
                }
            }

Steps to reproduce

1. Add style to your text
2. Select some text crossing several styles, for example select : " is a complicated mar".
3. Call createLink on this selection, specifying a buttonClass option.

Expected behavior: The button class you specified is added to the created a element.
Actual behavior: The button class is not added to the created a element.

Link to an example: https://jsfiddle.net/m63wpj5a/

Or even more simply:

Versions

• medium-editor: current
• browser:chrome (but any will do)