Custom "simpleLink" plugin erases links when switching from source to WYSIWYG mode

I followed this tutorial on creating custom plugins, because for our application we need a simple link plugin that only allows a simple link where the displayed text is the same as the URL, without any extra features or attributes.. It works great. However, when I tried to remove the built-in links plugin, the links get removed when you switch from source view back to WYSIWYG. The text remains, but it's no longer a link. e.g.

<p><a href="www.google.com">www.google.com</a></p>

became:

<p>www.google.com</p>

This occurs both if I remove the plugin completely, by removing it from our custom build, or if I just edit config.js to not display the links plugin by removing it from the toolbarGroups.

I finally got it to work by using the existing link plugin and just replacing the dialog with our custom dialog, but I don't like edit the plugin's code for obvious reasons. 

Is there something I'm missing from the custom plugin, to prevent the tag from being filtered out?