[collapsible.js] Parent pointers for maction content are wrong

[dpvc]

When content is enclosed in an maction, its parent pointers aren't redirected properly.

This originally showed up as mathjax/MathJax#1800, where mathsize from higher up in the tree was being applied twice in CommonHTML.

A test case is

<math mathsize="36pt">
<mi>x</mi>
<mi>x</mi>
<mi>x</mi>
<mi>x</mi>
<mi>x</mi>
</math>

when collapsible math is switched on.

[dpvc]

The issue210 branch includes the fix for this.

[dpvc]

==> Merged