cmd/go2avro: Struct default values differ from their type.fields defaults

[spaceweasel]

When a go struct has a slice or time field, the Avro defaults defined within each of the fields are [] and 0 respectively. However, the top-level struct default values are null and "0001-01-01T00:00:00Z"

A test to illustrate:

go2avro X
cmp stdout expect-stdout

go2avro example.com/foo/bar.X
cmp stdout expect-stdout

-- expect-stdout --
{
    "default": {
        "S": [],
        "T": 0
    },
    "fields": [
        {
            "default": [],
            "name": "S",
            "type": {
                "items": "string",
                "type": "array"
            }
        },
        {
            "default": 0,
            "name": "T",
            "type": {
                "logicalType": "timestamp-micros",
                "type": "long"
            }
        }
    ],
    "name": "X",
    "type": "record"
}
-- bar.go --
package bar

import "time"

type X struct {
  S []string
  T time.Time
}

-- go.mod --
module example.com/foo/bar

go 1.14

require github.com/heetch/avro v0.0.0-20200318154341-de261c0e4b7f // indirect
}

  > cmp stdout expect-stdout
  [diff -stdout +expect-stdout]
    {
        "fields": [
            {
                "default": {
  -                "S": null,
  -                "T": "0001-01-01T00:00:00Z"
  +                "S": [],
  +                "T": 0
                },
                "name": "C",
                "type": {
                    "fields": [
                        {
                            "default": [],
                            "name": "S",
                            "type": {
                                "items": "string",
                                "type": "array"
                            }
                        },
                        {
                            "default": 0,
                            "name": "T",
                            "type": {
                                "logicalType": "timestamp-micros",
                                "type": "long"
                            }
                        }
                    ],
                    "name": "C",
                    "type": "record"
                }
            }
        ],
        "name": "P",
        "type": "record"
    }

Not sure if this is related to #37

[spaceweasel]

Underlying issue same as #37