Homogenize treatment of params and meta in UnmarshalJSON (#52)

This change makes the treatment of params and meta the same, by
assigning a well-known pointer at first to detect if the unmarshaling
process overwrites it with an explicit nil, or it stays the same in
which it means that it was unset from the beginning.

jsonrpc2.go

@@ -85,10 +85,12 @@ func (r Request) MarshalJSON() ([]byte, error) {
 func (r *Request) UnmarshalJSON(data []byte) error {
 	r2 := make(map[string]interface{})
 
-	// Detect if the "params" field is JSON "null" or just not present
-	// by seeing if the field gets overwritten to nil.
+	// Detect if the "params" or "meta" fields are JSON "null" or just not
+	// present by seeing if the field gets overwritten to nil.
 	emptyParams := &json.RawMessage{}
 	r2["params"] = emptyParams
+	emptyMeta := &json.RawMessage{}
+	r2["meta"] = emptyMeta
 
 	decoder := json.NewDecoder(bytes.NewReader(data))
 	decoder.UseNumber()
@@ -112,9 +114,13 @@ func (r *Request) UnmarshalJSON(data []byte) error {
 		}
 		r.Params = (*json.RawMessage)(&b)
 	}
-	meta, ok := r2["meta"]
-	if ok {
-		b, err := json.Marshal(meta)
+	switch {
+	case r2["meta"] == nil:
+		r.Meta = &jsonNull
+	case r2["meta"] == emptyMeta:
+		r.Meta = nil
+	default:
+		b, err := json.Marshal(r2["meta"])
 		if err != nil {
 			return fmt.Errorf("failed to marshal Meta: %w", err)
 		}