Update the **class** of the **park* layer in a backward-compatible way. The class for `boundary=protected_area` parks is the lower-case of the [`protection_title`](http://wiki.openstreetmap.org/wiki/key:protection_title) value with blanks replaced by `_`. `national_park` is the class of `protection_title=National Park` and `boundary=national_park`. `nature_reserve` is the class of `protection_title=Nature Reserve` and `leisure=nature_reserve`. The class for other [`protection_title`](http://wiki.openstreetmap.org/wiki/key:protection_title) values is similarly assigned. Resolve https://github.com/openmaptiles/openmaptiles/issues/760
park
Docs
Read the layer documentation at http://openmaptiles.org/schema#park

