[BUG] Slice encoding drops empty values rather than encoding them as `nil` or default

[tombuente]

Current Behavior

r.ParseForm()
fmt.Println(r.PostForm["id"], len(r.PostForm["id"])) // [1  15] 3

var decoder = schema.NewDecoder()
var i Document
// r.PostForm is a map of our POST form values
err = decoder.Decode(&i, r.PostForm)
if err != nil {
    fmt.Println(err)
}

fmt.Println(i.ID, "Length", len(i.ID)) // ID [1 15] Length 2

When called with id=1, id=, id=15

The behavior is rather unfortunate when decoding values that matter when equal to "". For example, when two slices are related to each other.

type Document struct {
    RowID   []int64  `schema:"row_id"`
    RowName []string `schema:"row_name"`
}

Now RowID[0]and RowName[0] build a pair. But since gorilla/schema drops empty strings for some reason rather than settings it to Go's default for that value or using the default value defined by tags, the relationship breaks.

Expected Behavior

fmt.Println(i.ID, "Length", len(i.ID)) // ID [1 15] Length 3, add "" to struct slice

fmt.Println(i.ID, "Length", len(i.ID)) // ID [&addr nil &addr] Length 3, add nil to struct slice when field is pointer

[lll-lll-lll-lll]

@tombuente By passing true to the decoder's ZeroEmpty method, zero values for the field's type will be set.

https://github.com/gorilla/schema/blob/cd59f2f12cbdfa9c06aa63e425d1fe4a806967ff/decoder.go#L38-L48

[lll-lll-lll-lll]

r.ParseForm()
fmt.Println(r.PostForm["id"], len(r.PostForm["id"])) // [1  15] 3

var decoder = schema.NewDecoder()
// fix
decoder.ZeroEmpty(true)
var i Document
// r.PostForm is a map of our POST form values
err = decoder.Decode(&i, r.PostForm)
if err != nil {
    fmt.Println(err)
}

fmt.Println(i.ID, "Length", len(i.ID)) // ID [1 15] Length 2

[lll-lll-lll-lll]

@tombuente The specific processing is as follows. https://github.com/gorilla/schema/blob/cd59f2f12cbdfa9c06aa63e425d1fe4a806967ff/decoder.go#L355-L358