Make pay_url accept a block like link_to

Привет! I suggest to make pay_url accept block, just like link_to, to be able to construct link content in a more convenient way (template using haml):
```ruby
= pay_url @car_wash.payments.first.id, @car_wash.payments.first.amount do
  .pay-btn
    %div Оплатить с помощью
    %div= image_tag 'robokassa.jpg', size: '150x25'
```