bugfix: form_value use enum value not the key in active_record

[dskyu]

when i use enum feature in my active record, the value of the select in form is '' because the value option of the select is enum's value not the key.
eg: there is a Product model with enum status: {spare: 1, on_sale: 2, off_sale: 3}
without this commit,
the status field is always null, because rails_admin form builder can't find the correct value for the select
with this commit,

it will find the correct value, so the select value will be filled correctly

[luizpicolo]

This is fixed my error. Thanks.