Do not share options between Draw.Feature instances

Extending `this.options` in place makes the object shared between every instances of L.Draw.Feature.

This change prevents this behaviour.

I don't think this was intentional, and in my understanding of Leaflet conventions, `statics` are made for being shared, not `options`.

My use case is to add a measure tool on uMap using Leaflet.draw, while I also use the plugin for classic drawing on the map.

Thanks!

Yohan