fix: Empty map/record defaults as {} and primitive short form preservation

[StasPiv]

Problem

1. Empty map/record defaults serialize as [] instead of {}

PHP's json_encode([]) produces "[]", but Avro specification requires empty map/record values to be "{}".

Example:

// Input schema
{"type": "record", "name": "Test", "fields": [
  {"name": "meta", "type": {"type": "map", "values": "string"}, "default": {}}
]}

// Before fix (invalid)
"default": []

// After fix (correct)
"default": {}

2. Primitive types lose their short form

When parsing "string" (short form), the library set serialize_type_attribute=true, causing it to serialize back as {"type": "string"} (long form).

Example:

// Input
{"type": "record", "name": "Test", "fields": [
  {"name": "value", "type": "string"}
]}

// Before fix
"type": {"type": "string"}

// After fix
"type": "string"

Solution

1. Added serialize_default_value() method that recursively converts PHP arrays to stdClass for map/record types before JSON encoding
2. Changed serialize_type_attribute from true to false when parsing primitive types in short form