bug: []time.Duration not works

Example struct

```go
type MyStruct struct {
    BackoffPolicy []time.Duration `default:"...."`
}
```

Checked variants:
- `default:"{[\"10s\",\"1m\"]}"`
- `default:"[\"10s\",\"1m\"]"`
- `default:"[10s,1m]"`
- `default:"\"10s\",\"1m\""`
- `default:"10s,1m"`

all of them not works