toString generates poorly formatted defaults for bytes

Schema#toString and Protocol#toString both generate default values for "bytes" and "fixed" types.

According to docs:

Default values for bytes and fixed fields are JSON strings, where Unicode code points 0-255 are mapped to unsigned 8-bit byte values 0-255. Avro encodes a field even if its value is equal to its default.

The following schema

{
  "type" : "record",
  "name" : "TestRecord",
  "fields" : [ {
    "name" : "testFixed",
    "type" : {
      "type" : "fixed",
      "name" : "Code",
      "size" : 3
    },
    "default" : "\u0009\u0020\u00FF"
  } ]
}

If parsed and then again encoded to JSON would have "\t ÿ" value in "default".

It happens because `toString` implementations use `JsonGenerator` with default escape configs.