Use key when checking if session is in the Map.

[dimaa6]

Fixes #208. We already have session ID in session object, and can use it to make sure session is still in the Map keys. There is no need to check if the session object itself is in the Map values.

[TVolden]

Sorry for my slow response.
I have now merged the change.
Thanks for your contribution, it really means a lot to me.

Sincerely,
Thomas