Polyfill: Copy over attributes on the <math> node

[lukasbestle]

Hi Preet,

Thank you for creating this library. It works great as a polyfill!

I use both inline as well as block math elements on my pages. For that, I use the attribute display (an official attribute from the MathML spec) like so and react on it in my CSS:

<math display="block">
...
</math>

Unfortunately, the polyfill currently does not copy over attributes on the root element. I've fixed this in this PR. Let me know in case further changes are needed.