fix bug, redis session doesnt work
bug fix, session stored in redis cannot be deleted
bugfix, delete the sid if it's values is empty

* regenerate sid, if the old key doesn't exists, set the new one directly
Merge pull request #474 from pengfei-xue/develop

fix bug, redis session doesnt work
Merge branch 'develop' of git://github.com/astaxie/beego into develop