Fix divided_by behaviour when mathn is loaded

https://github.com/Shopify/liquid/issues/408, https://github.com/jekyll/jekyll/issues/2702

I don't think this is Liquid's fault, but this would be an easy way to "fix" it.

Not sure if this is a good idea. Maybe the "bug" is actually reasonable behaviour (if you include "mathn", this might be what you want). Thoughts?

@Shopify/liquid @parkr / cc @djacquel @albertogg