allow open() method to handle special session cookie lifetime value of zero

If the session cookie lifetime (session.cookie_lifetime) is zero, also set the cookie expiration time to zero, so that the special meaning of zero, "expire when browser closes," is preserved. (If this special meaning is not considered, the expiration time becomes time() + 0, or the current time, which causes the session to expire immediately.)
Merge pull request #12 from alanseiden/patch-1

allow open() method to handle special session cookie lifetime value of 0