This adds a bit more inconsistency to the library, but we didn't have complete consistency before. Almost all existing string enum values are lowercase (a couple are camelCase and one is dash-separated). The closure library isn't consistent either (with case for enum properties or values). I imagine this could be justified in saying someone could blindly use GeoJSON type values in places, but in the end, you'll need to read the docs before guessing right.
This allows us to cast single-part geometries to multi-part types before adding features to the target layer. This doesn't yet allow for drawing multi-part geometries with multiple parts. That can be handled separately.
