Add middot tests

I start with middot `·` to nbsp `&#160;` test for #332. Note:

1. Two middots convert to *one* nbsp. I don’t want crazy abuse of middots for styling purposes, like in the 90’s.
1. I convert the middot to the XML entity `&#160;` and not to `&nbsp;`. This allows to use the feature in atom feeds.

When this feature is implemented, I will add a new PR with the soft-hyphen feature. If I see how the middot is implemented, I can write it myself.