Fix for session handler overriding

This fixes a bug where setting a session.handler_id in your symfony
configuration didn't work, due to this bundle overriding it.

The setting is now only overridden if the handler_id is not null.
Merge branch 'master' into cheeky_solution
Reference parameter to ->get() fixed
Reference parameter to ->getByKey() fixed
Reference parameter to ->getMulti() fixed
Reference parameter to ->getMultiByKey() fixed
More surety that calls contains the correct function name added