Adds test for updating rack with resource_class_id

when resource_class_id is already there.
See #39 (https://github.com/tuskar/tuskar/issues/39)
Merge pull request #49 from petrblaho/test_for_rack_update_with_resource_class_id_issue_39

Adds test for updating rack with resource_class_id when resource_class_id is already there.