Fix JsonRpcRequest to be valid json (#130)

fix/types: JsonRpcParams should be valid JSON values

`undefined` is not valid JSON.

https://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
(Section 5):

    A JSON value can be an object, array, number, string, true, false, or null.

src/json.ts

@@ -177,10 +177,10 @@ export type JsonRpcError = OptionalField<
   'data'
 >;
 
-export const JsonRpcParamsStruct = optional(
-  union([record(string(), JsonStruct), array(JsonStruct)]),
-);
-export type JsonRpcParams = Infer<typeof JsonRpcParamsStruct>;
+export const JsonRpcParamsStruct: Struct<Json[] | Record<string, Json>, null> =
+  optional(union([record(string(), JsonStruct), array(JsonStruct)])) as any;
+
+export type JsonRpcParams = Json[] | Record<string, Json>;
 
 export const JsonRpcRequestStruct = object({
   id: JsonRpcIdStruct,