🐛 fix DataclassEncoder bug and add test case

tests/test_utils.py

@@ -0,0 +1,18 @@
+import json
+from utils import make_fake_message
+
+
+def test_dataclass_encoder():
+    from nonebot.utils import DataclassEncoder
+
+    MessageSegment = make_fake_message().get_segment_class()
+    ms = MessageSegment.node_custom(
+        "1234", "testtest", "test" + MessageSegment.image("url")
+    )
+    s = json.dumps(ms, cls = DataclassEncoder)
+    assert (
+        s
+        == '{"type": "node", "data": {"user_id": "1234", "nickname": "testtest", "content": [{"type": "text", "data": {"text": "test"}}, {"type": "image", "data": {"url": "url"}}]}}'
+    )
+    s1 = json.dumps("123", cls = DataclassEncoder)
+    assert s1 == '"123"'