Use active bool in overridden constructor

I noticed that the overridden constructor set Active=false no matter what was passed in. I also renamed the bool to active instead of notActive, to avoid a situation like Active=!notActive.
Remove Challenge from ParseAuthorizationHeader

I noticed that if the header is empty, we return null and let the caller issue a Challenge as they see fit. With tokens.Length < 2, though, we called Challenge and then returned null, in which case the caller probably calls Challenge again. Deferring Challenge to the caller in both cases seemed more consistent to me.
Merge pull request #1 from druttka/thoughts-on-active-and-challenge

Thoughts on active and challenge