Optional arguments

[jgraef]

Add support for optional arguments. We can either somehow interpret Option<T> arguments as optional or maybe add an attribute #[default] to tell the dispatcher-derive to use unwrap_or_default for this argument.

[sisou]

Additionally, when all arguments are optional (or the method doesn't take any arguments), the JSON-RPC spec says that the params property in the request object MAY also be omitted. This is currently not supported by this lib either.

[jsdanielh]

Resolved in #42.

[sisou]

Not resolved. Users still need to send null for all optional parameters. That is what the issue is originally about.

My change in #42 just allows omitting the params completely, not individual arguments.

[jsdanielh]

My bad, re-opening issue