Role mapping is only done once on user creation, so changes in AD never apply

Somewhat related to #58 in terms of AD values only being used when creating a user for the first time, the user roles and mappings are only processed once when a user first logs in and a WP user entry is created.

If there are subsequent changes in AD to a users roles etc... then these will not be updated in WP on subsequent login attempts so there is a sync issue.

We could make this behaviour default, with a return true/false filter to opt out of it, or vice versa, allowing sites to opt in to this behaviour instead. Given you already have to opt in to user role mapping via AD using a filter, I would vote for making this the default behaviour when using AD for managing roles.