[Issue apache#12486][Python Client]JsonSchema encoding is not idempot…

…ent (apache#12490)

* fix JsonSchema, copy data out to prevent modifying the reference object,
check keys before deleting them

* add unit test, but cannot test due to compilation failure for cpp lib

pulsar-client-cpp/python/pulsar/schema/schema.py

@@ -92,11 +92,16 @@ def _get_serialized_value(self, o):
 
     def encode(self, obj):
         self._validate_object_type(obj)
-        del obj.__dict__['_default']
-        del obj.__dict__['_required']
-        del obj.__dict__['_required_default']
-
-        return json.dumps(obj.__dict__, default=self._get_serialized_value, indent=True).encode('utf-8')
+        # Copy the dict of the object as to not modify the provided object via the reference provided
+        data = obj.__dict__.copy()
+        if '_default' in data:
+            del data['_default']
+        if '_required' in data:
+            del data['_required']
+        if '_required_default' in data:
+            del data['_required_default']
+
+        return json.dumps(data, default=self._get_serialized_value, indent=True).encode('utf-8')
 
     def decode(self, data):
         return self._record_cls(**json.loads(data))