Fix for collapsible sections width issue

With the fix introduced in #68, there was an issue were it was finding
the first element which wasn't a h2, when it should have been starting
at the first element and doing nothing if it was a h2. This commit fixes
that bug.

Also testing the headerlessContent function.
Merge pull request #70 from alphagov/fix-for-collapsible-manuals-width-issue

Fix for collapsible sections width issue