Nullable schema incorrect serialization

[shaeqahmed]

According to the avro spec, nullable fields in a schema should have the following shape:

{
                "name" : "access_key_id",
                "type" : [ "null", "string" ],
                "default" : null

When serializing an AvroField, in this library, the default value is serialized as the string literal "null" rather than the null JSON value. Looking through the code, this is caused by reusing the Schema type for this option, although the convention are a little different in reality (e.g. for types, null is written as a string). Currently this breaks compatibility when using this library with other Avro tools to read and write files.