Use str(Schema) rather than Schema.to_json to prevent fastavro fr…

…om raising exception `TypeError: unhashable type: 'mappingproxy'`

Fixes confluentinc#1156

src/confluent_kafka/avro/serializer/message_serializer.py

@@ -20,6 +20,7 @@
 # derived from https://github.com/verisign/python-confluent-schemaregistry.git
 #
 import io
+import json
 import logging
 import struct
 import sys
@@ -79,7 +80,7 @@ def __init__(self, registry_client, reader_key_schema=None, reader_value_schema=
     # Encoder support
     def _get_encoder_func(self, writer_schema):
         if HAS_FAST:
-            schema = writer_schema.to_json()
+            schema = json.loads(str(writer_schema))
             parsed_schema = parse_schema(schema)
             return lambda record, fp: schemaless_writer(fp, parsed_schema, record)
         writer = avro.io.DatumWriter(writer_schema)