fix: add test case for serializing issue

tests/serializer_uuid.rs

@@ -0,0 +1,42 @@
+use async_graphql::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize)]
+#[repr(transparent)]
+#[serde(transparent)]
+struct AssetId(pub uuid::Uuid);
+
+async_graphql::scalar!(AssetId);
+
+#[tokio::test]
+/// Test case for
+/// https://github.com/async-graphql/async-graphql/issues/603
+pub async fn test_serialize_uuid() {
+    let generated_uuid = uuid::Uuid::new_v4();
+
+    struct Query {
+        data: AssetId,
+    }
+
+    #[Object]
+    impl Query {
+        async fn data(&self) -> &AssetId {
+            &self.data
+        }
+    }
+
+    let schema = Schema::new(
+        Query {
+            data: AssetId(generated_uuid),
+        },
+        EmptyMutation,
+        EmptySubscription,
+    );
+    let query = r#"{ data }"#;
+    assert_eq!(
+        schema.execute(query).await.data,
+        value!({
+            "data": generated_uuid.to_string(),
+        })
+    );
+}