[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.

schema/decoder.go

Lines 38 to 48 in cd59f2f

	// ZeroEmpty controls the behaviour when the decoder encounters empty values
	// in a map.
	// If z is true and a key in the map has the empty string as a value
	// then the corresponding struct field is set to the zero value.
	// If z is false then empty strings are ignored.
	//
	// The default value is false, that is empty values do not change
	// the value of the struct field.
	func (d *Decoder) ZeroEmpty(z bool) {
	d.zeroEmpty = z
	}

[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.

schema/decoder.go

Lines 355 to 358 in cd59f2f

	if value == "" {
	if d.zeroEmpty {
	items = append(items, reflect.Zero(elemT))
	}