iPay88 acknowledge should only requery when the status is success

@bslobodin @bizla 

When iPay88 returns from an error state (such as cancellation), acknowledge returns false. They still pass us a signature, so we can still acknowledge. This fixes that by only calling `requery` when the status is success.
What will make it easier to read? Before `acknowledge` would return false if `success?` returned false. We want it to return true even if `success?` is false, and if `success?` is true we want to check `requery`.

I can redo it as?

``` ruby
def acknowledge
  return false unless secure?
  return true unless success?
  
  requery == '00'
end
```