Option to clear auth header on redirects

Added an option that will allow the FollowRedirects middleware to clear authorization headers on redirects. The HTTP spec doesn't seem to specify whether particular headers should be omitted/included in redirects (http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3) so it seems reasonable for an option like this to be built into the middleware