fix to_xml for unprocessable_entity responses

[kzink]

Rails 7+ has removed ActiveModel::Errors#to_xml which responders was relying on to emit :unprocessable_entity errors in xml format. You'll see something like this if it tries to render the error collection into a response:

undefined method `bytesize' for #<ActiveModel::Error attribute=name, type=blank, options={}>

I cobbled together a fix for this by pulling out the formatting method used in Rails 6 to restore the functionality. Unsure if this works in Rails <6 so it might be worth gating with a conditional.