BUG Fixes #125. Error viewing WorkflowInstance in GridField.

When the next action is an AssignUsersToWorkflowAction, Member relations
are wiped. But users on the previous action still need to see the
WorkflowInstance in the PendingObjects GridField after editing+saving.
Merge pull request #134 from phptek/issue/125

BUG Fixes #125. Error viewing WorkflowInstance in GridField.