SAML `auto_link_ldap_user` trying to find user by uid instead of DN

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/22546

If auto_link_ldap_user is enabled, SAML authentication should try to find LDAP users and create their user and identity. SAML generally uses the user's DN as the 'uid' from omniauth. We correctly look for existing LDAP identities by DN, but we incorrectly look up LDAP users by uid if the identity doesn't exist. The bug exists in OAuth::User in ldap_person method.

cc/ @patricio