fix: Set default value for form fields

- Use provided default value in struct tags when binding a request input to struct for validation.

Fixes: How to apply default value if empty value provided by client during model binding? gin-gonic#4042, #13042df, #a41721a

binding/form_mapping.go

@@ -261,6 +261,9 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 
 		if len(vs) > 0 {
 			val = vs[0]
+			if val == "" {
+				val = opt.defaultValue
+			}
 		}
 		if ok, err := trySetCustom(val, value); ok {
 			return ok, err

binding/form_mapping_test.go

@@ -152,6 +152,15 @@ func TestMappingForm(t *testing.T) {
 	assert.Equal(t, 6, s.F)
 }
 
+func TestMappingFormWithEmptyToDefault(t *testing.T) {
+	var s struct {
+		F string `form:"field,default=DefVal"`
+	}
+	err := mapForm(&s, map[string][]string{"field": {""}})
+	require.NoError(t, err)
+	assert.Equal(t, "DefVal", s.F)
+}
+
 func TestMapFormWithTag(t *testing.T) {
 	var s struct {
 		F int `externalTag:"field"`