release pool connection restores pool user configuration

Discussed potential for this in #833 whereby a connection that is checked out from the pool then has `changeUser` called it would be marked for purging when it was released.  Instead during release `changeUser` is called again to restore it back to the pool configuration before it's released back into the pool.

I've updated the test that covered restoreUser and all tests are passing locally, but I couldn't find a jshint or styleguide so I'm fully open to changes before merging.