circe: Filter out flat optionals

modules/core/src/main/scala/jsonrpclib/internals/RawMessage.scala

@@ -63,7 +63,7 @@ private[jsonrpclib] object RawMessage {
           "params" -> msg.params.asJson,
           "error" -> msg.error.asJson,
           "id" -> msg.id.asJson
-        ) ++ {
+        ).filterNot(_._2.isNull) ++ {
           msg.result match {
             case Some(Some(payload)) => List("result" -> payload.asJson)
             case Some(None)          => List("result" -> Json.Null)

modules/core/src/test/scala/jsonrpclib/RawMessageSpec.scala

@@ -7,12 +7,13 @@ import jsonrpclib.CallId.NumberId
 import jsonrpclib.OutputMessage.ResponseMessage
 import io.circe.Json
 import com.github.plokhotnyuk.jsoniter_scala.circe.JsoniterScalaCodec._
+import io.circe.syntax._
 
 object RawMessageSpec extends FunSuite {
   test("json parsing with null result") {
     // This is a perfectly valid response object, as result field has to be present,
     // but can be null: https://www.jsonrpc.org/specification#response_object
-    val rawMessage = readFromString[Json](""" {"jsonrpc":"2.0","result":null,"id":3} """.trim)
+    val rawMessage = readFromString[Json](""" {"jsonrpc":"2.0","id":3,"result":null}""".trim)
       .as[RawMessage]
       .fold(throw _, identity)
 
@@ -31,4 +32,40 @@ object RawMessageSpec extends FunSuite {
     ) &&
     assert(invalidRawMessage.toMessage.isLeft, invalidRawMessage.toMessage.toString)
   }
+
+  test("request message serialization") {
+    val input: Message = InputMessage.RequestMessage("my/method", CallId.NumberId(1), None)
+    val expected = """{"jsonrpc":"2.0","method":"my/method","id":1}"""
+    val result = writeToString(input.asJson)
+
+    assert(result == expected, s"Expected: $expected, got: $result")
+  }
+
+  test("notification message serialization") {
+    val input: Message = InputMessage.NotificationMessage("my/method", None)
+    val expected = """{"jsonrpc":"2.0","method":"my/method"}"""
+    val result = writeToString(input.asJson)
+
+    assert(result == expected, s"Expected: $expected, got: $result")
+  }
+
+  test("response message serialization") {
+    val input: Message = OutputMessage.ResponseMessage(CallId.NumberId(1), Payload.NullPayload)
+    val expected = """{"jsonrpc":"2.0","id":1,"result":null}"""
+    val result = writeToString(input.asJson)
+
+    assert(result == expected, s"Expected: $expected, got: $result")
+  }
+
+  test("error message serialization") {
+    val input: Message = OutputMessage.ErrorMessage(
+      CallId.NumberId(1),
+      ErrorPayload(-32603, "Internal error", None)
+    )
+    val expected = """{"jsonrpc":"2.0","error":{"code":-32603,"message":"Internal error","data":null},"id":1}"""
+    val result = writeToString(input.asJson)
+
+    assert(result == expected, s"Expected: $expected, got: $result")
+  }
+
 }