Changes

Wrap primitive and array parameter into an array as in JSON-RPC 2.0

Add missing quotes around body in JSON

Gson was parsing this malformed JSON, but it wasn't actually correct
without this escaping.

Before the message would look like:

```json
{"seq":10,"type":"response","request_seq":4,"command":"foo","success":true,body:{"allThreadsContinued":false}}
```

Now it looks like:

```json
{"seq":10,"type":"response","request_seq":4,"command":"foo","success":true,"body":{"allThreadsContinued":false}}

```

Fix typo in test name

Void return types need to always be null value

Void type in Java cannot be instantiated, therefore if a
type is declared as Void the fromJson should always set that
value to null.

If you want to have a generic return type that allows non-null
and null results, use a type other than Void as the return type.

Fixes #721

Update docs for 0.21.0 release

Prepare repo for 0.21.0 release