Fix the jsonrpc adapter to reference the method correctly

/to @ritch 

See https://github.com/strongloop/strong-remoting/issues/124

To enable the jsonrpc adapter, add the following to server/boot/json-rpc.js

```js
module.exports = function(app) {
  app.use('/services', app.handler('jsonrpc'));
}
```

It will expose `POST /services/my-model/jsonrpc` as jsonrpc endpoints.

To invoke the method:

```sh
curl -X POST -d '{"jsonrpc": "2.0", "method": "find", "id": "1"}' http://127.0.0.1:3000/services/my-model/jsonrpc -H 'Accept: application/json' -H 'Content-Type: application/json' --verbose 
```