Schema generation of defaults is incompatible with Avro schema

[DanWertheimer]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything

Bug

• py2k version: 1.9.1
• Python version: Any
• Operating System: Any

Expected Result

When generating a Schema with a default, it should look like:

{"name": "xxx", "type": ["null", "boolean"], "default": null}

Actual Result

{"name": "xxx", "type": "boolean", "default": null}