Fix for relative redirects

I have made a minor fix to support relative redirects, i.e. when the `response['location']` is just a path without the host.   I have added some tests which are using the real world example where I found the problem.  Ruby is not my language so I have no idea if I have fixed the problem in the most optimal way.

The example URL is [http://www.auc.edu.au/devworld](http://www.auc.edu.au/devworld) which redirects to [http://auc.edu.au/devworld/about/](http://auc.edu.au/devworld/about/).  The link checker without the fix generates a connection error.  As I say, Ruby isn't my language so I don't know how to mock up a test for relative redirects.

Feel free to do what you will with my changes.  Thanks for the link checker.