Missing support for slices with BindUnmarshaler elements

[asmeikal]

Currently there is no support for slices that have elements implementing the BindUnmarshaler interface, but there is support for slices that implement directly BindUnmarshaler (thanks to #3970 and #3971).

This is caused by setArray always calling setWithProperType on its elements.

This is a test reproducing the problem:

package test

import (
	"errors"
	"github.com/gin-gonic/gin/binding"
	"github.com/stretchr/testify/assert"
	"net/url"
	"strings"
	"testing"
)

type customUnmarshalParamType struct {
	Protocol string
	Path     string
	Name     string
}

func (f *customUnmarshalParamType) UnmarshalParam(param string) error {
	parts := strings.Split(param, ":")
	if len(parts) != 3 {
		return errors.New("invalid format")
	}
	f.Protocol = parts[0]
	f.Path = parts[1]
	f.Name = parts[2]
	return nil
}

func TestSliceOfBindUnmarshal(t *testing.T) {
	var s, expected struct {
		FileData []customUnmarshalParamType `form:"data"`
	}
	form := url.Values{"data": {`file:/foo:happiness`, `http:/bar:sadness`}}
	err := binding.MapFormWithTag(&s, form, "form")
	assert.Nil(t, err)
	expected.FileData = []customUnmarshalParamType{
		{
			Protocol: "file",
			Path:     "/foo",
			Name:     "happiness",
		},
		{
			Protocol: "http",
			Path:     "/bar",
			Name:     "sadness",
		},
	}
	assert.Equal(t, expected, s)
}

In this case the mapping fails with a json.SyntaxError with a value of invalid character 'i' in literal false (expecting 'a'), since the default behaviour for struct fields is to parse them as JSON.

This happens for both the latest tagged version (v1.10.1) and using the latest commit from master.