refactor Field to have defaultTag field which value is from parent struct

field.go

@@ -16,6 +16,7 @@ var (
 type Field struct {
 	value      reflect.Value
 	field      reflect.StructField
+	defaultTag string
 }
 
 // Tag returns the value associated with key in the tag string. If there is no
@@ -92,7 +93,7 @@ func (f *Field) Set(val interface{}) error {
 //
 // It panics if field is not exported or if field's kind is not struct
 func (f *Field) Fields() []*Field {
-	return getFields(f.value, DefaultTagName)
+	return getFields(f.value, f.defaultTag)
 }
 
 // Field returns the field from a nested struct. It panics if the nested struct

structs.go

@@ -224,6 +224,7 @@ func (s *Struct) FieldOk(name string) (*Field, bool) {
 	return &Field{
 		field:      field,
 		value:      s.value.FieldByName(name),
+		defaultTag: s.TagName,
 	}, true
 }