Pagseguro invalid HTTP code

@odorcicd @bslobodin 

Instead of trying to parse the XML, I'm checking for the response code and raising the error with the content of the body and the http code.
I think this may be better?
```ruby
exception = assert_raise(StandardError) {PagSeguro::Notification.new(notification_data)}
assert_equal "Response body: 'Not found' Response code: 404", exception.message
```