Make _default.default bound method; fix #323

Previously, our JSON encoder monkey patch was to an unbound method.
This resulted in throwing a misleading `TypeError` when trying to encode
something non-encodable.

pottery/monkey.py

@@ -55,7 +55,7 @@ def _default(self: Any, obj: Any) -> Union[Dict[str, Any], List[Any], str]:
     return return_value
 
 import json  # isort:skip
-_default.default = json.JSONEncoder.default  # type: ignore
+_default.default = json.JSONEncoder().default  # type: ignore
 json.JSONEncoder.default = _default  # type: ignore
 
 _logger.info(

tests/test_monkey.py

@@ -0,0 +1,20 @@
+# --------------------------------------------------------------------------- #
+#   test_monkey.py                                                            #
+#                                                                             #
+#   Copyright © 2015-2021, Rajiv Bakulesh Shah, original author.              #
+#   All rights reserved.                                                      #
+# --------------------------------------------------------------------------- #
+
+
+import json
+
+from tests.base import TestCase
+
+
+class MonkeyKeyTests(TestCase):
+    def test_json_encoder(self):
+        try:
+            json.dumps(object())
+        except TypeError as error:
+            assert str(error) == \
+                'Object of type object is not JSON serializable'