Always omit params member from request when empty

With this commit, the JSON encoding of `Request` always omits the params
member when calling `Conn.Call`, `Conn.DispatchCall`, or `Conn.Notify`
with the `params` argument set to `nil`. This change also removes the
`OmitNilParams` call option that was added in commit 8012d496 (#62).

As of this commit, if users desire to send a JSON-RPC request with a
`params` value of `null`, then they may do so by explicitly setting the
`params` argument of `Conn.Call`/`Conn.DispatchCall`/`Conn.Notify` to
`json.RawMessage("null")`.

jsonrpc2_test.go

@@ -321,3 +321,10 @@ func serve(ctx context.Context, lis net.Listener, h jsonrpc2.Handler, streamMake
 		jsonrpc2.NewConn(ctx, streamMaker(conn), h, opts...)
 	}
 }
+
+func rawJSONMessage(v string) *json.RawMessage {
+	b := []byte(v)
+	return (*json.RawMessage)(&b)
+}
+
+var jsonNull = json.RawMessage("null")