Test it. [WIP]

Turns out that it seemed to be fixed, but that was not really the case. Dragging and zooming was ok, but calling `setView` did not sync, which is perfectly logical in retrospect.

This adds some tests using mocha/chai and fixes for the problems found. 

Not ready yet though.

If you don't like looking at my PRs all the time, I'm more than happy welcoming the project in the [Leaflet-extras](https://github.com/leaflet-extras/leaflet-providers) org.