Fix multiword enum cases encoding and decoding

[evil159]

What does this pull request do?

Multi-word enum cases were not encoded/decoded properly - instead of the correct form e.g. some-value it would be some_value which would cause a PlatformException.

What is the motivation and context behind this change?

https://github.com/mapbox/mapbox-maps-flutter/issues/500, https://github.com/mapbox/mapbox-maps-flutter/issues/455, https://github.com/mapbox/mapbox-maps-flutter/issues/161, https://mapbox.atlassian.net/browse/MAPSFLT-206

Pull request checklist:

• [x] Add a changelog entry.
• [ ] Write tests for all new functionality. If tests were not written, please explain why.
• [ ] Add documentation comments for any added or updated public APIs.

[bf-vs]

LGTM but why not use enhanced enums? It would be a cleaner solution.

[evil159]

LGTM but why not use enhanced enums? It would be a cleaner solution.

Our code generation tool - Pigeon, doesn't support them.