skip params when it is `None`

[hxzhao527]

when I use this lib to make a call to aria2c system.listMethods, I find the params serialized to null even when the method defined without args.

use jsonrpc_core_client::transports::http;
use jsonrpc_core::Result;
use jsonrpc_derive::rpc;

/// Rpc trait
#[rpc]
pub trait Rpc {
	/// Returns a protocol version
	#[rpc(name = "system.listMethods")]
	fn list_methods(&self) -> Result<Vec<String>>;
}

#[tokio::main]
async fn main() -> Result<()>{
	let client = http::connect::<gen_client::Client>("http://127.0.0.1:6800/jsonrpc").await.unwrap();
	println!("{:?}", client.list_methods().await.unwrap()); 

	Ok(())
}

the code will send {"jsonrpc":"2.0","method":"system.listMethods","params":null,"id":0} to aria2 and then got HTTPError: HTTP Error 500: Internal Server Error

if skip the params when it is None, things go right.

/// Represents jsonrpc request which is a method call.
#[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
pub struct MethodCall {
	/// A String specifying the version of the JSON-RPC protocol.
	pub jsonrpc: Option<Version>,
	/// A String containing the name of the method to be invoked.
	pub method: String,
	/// A Structured value that holds the parameter values to be used
	/// during the invocation of the method. This member MAY be omitted.
	#[serde(default = "default_params")]
	#[serde(skip_serializing_if="omit_params")]
	pub params: Params,
	/// An identifier established by the Client that MUST contain a String,
	/// Number, or NULL value if included. If it is not included it is assumed
	/// to be a notification.
	pub id: Id,
}

fn omit_params(param: &Params) -> bool {
	&Params::None == param
}

so could we add this annotation to skip serializing when it is None?