fix(server): Return ping's payload through the response pong

Closes enisdenjo#117

src/server.ts

@@ -586,7 +586,16 @@ export function makeServer<E = unknown>(options: ServerOptions<E>): Server<E> {
             return;
           }
           case MessageType.Ping: {
-            await socket.send(stringifyMessage({ type: MessageType.Pong }));
+            await socket.send(
+              stringifyMessage(
+                message.payload
+                  ? { type: MessageType.Pong, payload: message.payload }
+                  : {
+                      type: MessageType.Pong,
+                      // payload is completely absent if not provided
+                    },
+              ),
+            );
             return;
           }
           case MessageType.Pong:

src/tests/server.ts

@@ -662,6 +662,26 @@ describe('Ping/Pong', () => {
     });
   });
 
+  it("should return ping's payload through the pong", async () => {
+    const { url } = await startTServer();
+
+    const client = await createTClient(url);
+
+    client.ws.send(
+      stringifyMessage({
+        type: MessageType.Ping,
+        payload: { iCome: 'back' },
+      }),
+    );
+
+    await client.waitForMessage(({ data }) => {
+      expect(parseMessage(data)).toEqual({
+        type: MessageType.Pong,
+        payload: { iCome: 'back' },
+      });
+    });
+  });
+
   it('should not react to a pong', async () => {
     const { url } = await startTServer();