Minor bugfix for rel attribute

When linkify is called on some html where the &lt;a&gt; tags already have a rel attribute, it is replaced with "nofollow". 
Additionally, if nofollow already one of the values for rel, e.g. if rel="tt nofollow", the value is also replaced with just rel="nofollow".
This pull request contains a test that exposes the bug and a fix (I hope).