Reset audited_user when yield raises exception

Because if it's not set to nil, all audits made from the sweeper will
use the user from this block instead of current_user, until
Audit.as_user is called again.
Merge branch 'reset_audited_user_when_yield_raises_exception' of https://github.com/flugsio/audited into flugsio-reset_audited_user_when_yield_raises_exception

* 'reset_audited_user_when_yield_raises_exception' of https://github.com/flugsio/audited:
  Reset audited_user when yield raises exception