Correctly format errors when sent to the wire

Instances of Blather::StanzaError and Blather::StreamError had an
implementation of `to_xml` that does not take an argument. A problem
with the previous implementation is that the following statement will
throw an exception:

```ruby
client.write Blather::StanzaError.new(stanza, 'registration-required',
:cancel)
```

The reason is that `client.write` calls `stream.send`, which in turn
calls `#to_xml` with an argument on the stanza:

```ruby
stanza.to_xml(:save_with => Nokogiri::XML::Node::SaveOptions::AS_XML)
```

The difference between `to_xml` with the formatting option and `to_s`
is whitespace.  The string produced with `to_xml(:save_with => AS_XML)`
strips off any whitespace between tags whereas `to_s` keeps the
whitespace between tags intact.

Fixes #144
Guard-rspec wants a full command now