HELIUM: toJSON should throw unchecked exception

The toJSON methods in Array, Dictionary, Result and Document should all throw an unchecked exception if they fail because some string cannot be translated from UTF-8 (Fleece) encoding, to UTF-16 (Java String encoding)

The exception is unchecked in the 3.1 branch ONLY. This avoids changing the API.

Currently all except Document are annotated as @NonNull, when, in fact, they can return null. Document is annotated as @Nullable and should stay that way. All the others should be annotated @NonNull (not a breaking change) but changed to throw an IllegalStateException