fixes gitlab.com issue #506 - Asignee for Issuables not checked
Created by: MrMarvin
As seen in https://gitlab.com/gitlab-org/gitlab-ce/issues/506, the assignee of an Issue and/or MergeRequest wasn't checked for permission on the project.
This PR comes with enhanced specs in both Issue and Mergerequest. These
two models are the only users of Issuable as far as I see. The
validation method needs to be overwritten in both of them as the
iactual implementation of permissions is unknown in the Issuable Concern
module.