Ironing JSONSerialization quirks

[TizianoCoroneo]

Removed a workaround in the jsonValue property of Optional: the solution was to make NSDictionary conform to JSONEncodable.

Also fixed an issue where trying to serialize .some(NSNull()) would result in a crash.

BREAKING CHANGE (?): This slightly changes the result of the JSON serialization, because instead of taking a NSDictionary, casting to Dictionary and then calling jsonValue on it, it directly uses the NSDictionary as the JSONValue itself.
Previously, this resulted in having Bool values encoded as NSNumbers, while now they are encoded as a true or false JSON constants. IMHO, this is slightly more correct.

[designatednerd]

I agree having true/false is more correct, so I'm fine with that change.

In theory this shouldn't be breaking, since it shouldn't produce different results under the hood with the exception of true/false vs 1/0.