Avoid removal of attributes used by policy engine

Avoid removal of attributes used by policy engine

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