Prevent duplicate id in heading and inserted a tag

[brettbatie]

If the heading tag already has the exact same id as what will be inserted into the tag, do not add the id to the tag. This helps with plugins like SimpleTOC which adds an id to all of the heading tags.

[vyskoczilova]

Hi! Thanks for the PR! How I should test it?

[brettbatie]

You can manually test it by adding an h2 tag to a post and then give the h2 tag an id that matches the id that add-anchor-links will generate on the a tag. Then view the published post to confirm that the inserted a tag does not have an id added because the id is already on the h2 tag.

Below is the output that was generated before this pull request (notice the duplicate id="faq"):

<h2 id="faq"><a href="#faq" aria-hidden="true" class="aal_anchor"  id="faq"><svg>...</svg></a>faq</h2>

With the pull request the output will be:

<h2 id="faq"><a href="#faq" aria-hidden="true" class="aal_anchor"><svg>...</svg></a>faq</h2>

You can also confirm that the id is still added to the a tag when a different id is on the h2 tag or when the h2 tag does not have an id.

[vyskoczilova]

Thanks again! Sorry for the delay, but I'm finishing my PhD and I'm running out of my time. Will check it in October.