Fix invalid XML requests

The commit messages include the details around the issues. In short, a component attribute in line nodes is not valid, nor is a request node without a url node. The generated request was not validating against the Notifier 2.3 XSD schema and said requests (while returning a 200 from the API, which I've notified Airbrake of) were never showing up in Airbrake.

I've only just begun using Airbrake, so I'm not sure if these were due to changes in the API. However with these updates things are working great. Thanks for creating this great lib!