Cannot parse non-numerical ID

Per the json-rpc spec, the `"Id"` in the request can be a number, a string or NULL, but this library throws an error when using a string:

```
{
"jsonrpc": "2.0",
"id": 0,
"error": {
"code": -32700,
"message": "unmarshaling request: json: cannot unmarshal string into Go struct field request.id of type int64"
}
}
```

Additionally, the response breaks the spec too because `If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be Null.`.