MessageExtras to comply with TM2i

[QuintinWillison]

Originally specified: ably/ably-java#580

And fixed in: ably/ably-java#581

[QuintinWillison]

@sacOO7 it may be that this issue can simply be closed - can you validate if this has been fixed, or create a PR to fix accordingly. Thanks.

[sacOO7]

Findings with respect to ably/ably-java#581

• MessageExtras does not check for null data (JToken) in the constructor.
• DeltaExtras does not check for null Format and From values.
• ToJson() method can be simplified, data (Jtoken) already contains Delta property.

[sacOO7]

Note - As per https://docs.ably.io/client-lib-development-guide/features/#TM2i, JsonObject is specified as optional (nullable) value.
Need to confirm if its null or non-null.

[marto83]

I leave it null because this way we don't serialise an empty json object ({}) when sending the message to the server. Same for From and Format. If the client are never going to set them we can probably make the constructor private.

Agree about ToJson() now everything is immutable we can simplify it.

[sacOO7]

@QuintinWillison we will need your inputs on this #422 (comment)