Fix FormBuilder#inputs content capturing

Without this change this would return an empty fieldset

```ruby
ActiveAdmin.register Member do
  form do |f|
    f.inputs 'Authentication' do
      f.input :email
      
      # when the following is false the block returns nil
      unless f.object.persisted?
        f.input :password
        f.input :password_confirmation
      end
    end
  end  
end
```

**edit:**

A quick workaround (yet not definitive), thanks to @thbar:


```ruby
ActiveAdmin.register Member do
  form do |f|
    f.inputs 'Authentication' do
      f.input :email
      
      # when the following is false the block returns nil
      unless f.object.persisted?
        f.input :password
        f.input :password_confirmation
      end
      f.form_buffers.last # <<< add this
    end
  end  
end
```