Merge pull request #442 from sebastienluquet/fix_blank_session_cookie

Do not raise ArgumentError, key cannot be blank if user forge a blank session cookie
Merge pull request #443 from sebastienluquet/fix_blank_session_cookie

add missing method since DalliStore < AbstractStore not CacheStore
restore nil namespace
Merge pull request #444 from sebastienluquet/allow_nil_namespace

restore nil namespace
better fix for blank session cookie
Do not invalidate current session