Nested structs should not require the use of the `none` format.

Currently, to decode nested structs properly, you must use the `none` format option. 

```go
type Nested struct {
	First  string `fixed:"1,3"`
	Second string `fixed:"4,6"`
}

type Test struct {
	First  string `fixed:"1,3"`
	Second Nested `fixed:"4,9,none"`
}
```

This is a breaking change and should be released as part of a major version bump.
