restoring pull request fixing #1623

Because i though it will not be merged, i checkout-ed auth.py in my another pull request.
As for that pull request is merged, my previous fix of auth.py to add 'MD5-sess' is deleted in master branch.
https://github.com/kennethreitz/requests/commit/3501ea463432400c77479dddedab3b7af967b618
(it's original commit)
Sorry for my mistake and sorry for confusing :(
This is pull request restoring previous fixes.
thanks.