Reuse mutant instance on re-adding to map
See #75 (closed) - the "new" billing is based on
google.maps.Map instances, not on tiles nor map views.
This can create a slight problem when an implementation of GoogleMutant keeps removing and adding GoogleMutant instances to a Leaflet map - each of those will generate one more billing item.
this._mutant can be reused. Ideally it should be a simple hack in the
_initMutant method, check if the value exists, then return it directly. All the mutant events are handled outside
_initMutant so those shouldn't be a problem. Merge requests welcome.