Fix binding of untagged struct fields (#812)

* Add failing test

A BindUnmarshaler struct with no tag is not decoded properly.

* Fix binding of untagged structs

bind.go

@@ -95,7 +95,7 @@ func (b *DefaultBinder) bindData(ptr interface{}, data map[string][]string, tag
 		if inputFieldName == "" {
 			inputFieldName = typeField.Name
 			// If tag is nil, we inspect if the field is a struct.
-			if structFieldKind == reflect.Struct {
+			if _, ok := bindUnmarshaler(structField); !ok && structFieldKind == reflect.Struct {
 				err := b.bindData(structField.Addr().Interface(), data, tag)
 				if err != nil {
 					return err
@@ -185,16 +185,24 @@ func unmarshalField(valueKind reflect.Kind, val string, field reflect.Value) (bo
 	}
 }
 
-func unmarshalFieldNonPtr(value string, field reflect.Value) (bool, error) {
+// bindUnmarshaler attempts to unmarshal a reflect.Value into a BindUnmarshaler
+func bindUnmarshaler(field reflect.Value) (BindUnmarshaler, bool) {
 	ptr := reflect.New(field.Type())
 	if ptr.CanInterface() {
 		iface := ptr.Interface()
 		if unmarshaler, ok := iface.(BindUnmarshaler); ok {
-			err := unmarshaler.UnmarshalParam(value)
-			field.Set(ptr.Elem())
-			return true, err
+			return unmarshaler, ok
 		}
 	}
+	return nil, false
+}
+
+func unmarshalFieldNonPtr(value string, field reflect.Value) (bool, error) {
+	if unmarshaler, ok := bindUnmarshaler(field); ok {
+		err := unmarshaler.UnmarshalParam(value)
+		field.Set(reflect.ValueOf(unmarshaler).Elem())
+		return true, err
+	}
 	return false, nil
 }