Fix Form::label() missing for attribute #1759

Signed-off-by: Kenji Suzuki <kenji.uui@gmail.com>
Fix Form::lable() logic

Signed-off-by: Kenji Suzuki <kenji.uui@gmail.com>
Merge pull request #1761 from kenjis/fix_form_label

Fix Form::label() missing for attribute #1759