Issue description:

After initializing and using simple_bind_s everything works great. But when it comes to unbind_s function it gives exception of AttributeError which is resulted to "SimpleLDAPObject has no attribute '_l'"

Steps to reproduce:
initialize ldap, bind_s, then unbind will result to this error.

Operating system: macOS 10.14
Python version: 3.6
python-ldap version: 3.1.0