(cherry picked from commit 0b2e3e8)

It's ok if more notifications got emitted than we expected. Just don't
let it break the test.

(cherry picked from commit ef933c9)

Not sure what other JSON-RPC JavaScript clients do, but rpc-websockets
sets request object `params` to `null`. Because the key exists we can't
make `has-params` method return _False_. But because it's undefined
it's ok not to use it.

(cherry picked from commit f16d751)

Correctly translate it into JSON `null`.

Sometimes 1+ extra messages could be emitted before socket closes. It's
ok.

Now, even less of the boilerplate!

Unfortunately, to do full-fledged unmarshalling I need PR
tadzik/JSON-Unmarshal#33 to be merged. Until then I incorporated the
module as `Cro::RPC::JSON::Unmarshal` with the necessary modifications.

Looks lik `m:s/.../` kind of regex causes it.