Fixes bug1025150
Ensures that each attribute which is used by the policy engine is not
removed by the plugin when the 'fields' query parameter is specified.
This could be better achieved by not having the plugin perform this
selection, but as it is part of the plugin interface, it's probably better
not to introduce changes which alter it.
Change-Id: I68ab0998b7113f06da5df74ccd44e390640de475
29707cf347