delegate to_xml (with options) to the node

Instead of defining `to_xml` as `node.to_s` call `node.to_xml`, which also accepts an options hash to control the output.  With this change the `to_xml` method for errors behaves just like any other stanza's `to_xml` method.
Could you please include this in the test? Maybe...

```ruby
it 'accepts formatting options' do
  # Without spaces
  string = ...
```