Created by: kevinushey
This PR should help resolve https://github.com/mathjax/MathJax/issues/1584.
I considered using Array.isArray, but because this is an ES5 feature it might not work with older browsers. This could be implemented with a simple polyfill (e.g. just by adding this method to the Array prototype) but not sure if this is a desirable solution for MathJax.