fix(angular-mocks): use copy of mock data in $httpBackend

fix(angular-mocks): use copy of mock data in $httpBackend

Copy mock data returned from the mock $httpBackend.
This prevents modifications to the response from affecting future responses.
Previously, this misbehavior was being mitigated by the deep copy in $resource, but that no longer exists.

https://github.com/angular/angular.js/issues/5265