added test cases for plug caching behaviour at scene root
added test cases for plug caching behaviour at scene root

A problem was recently discovered where if an attributes plug was pulled on at the scene root, all subsequent pulls would incorrectly return an empty compound object. These tests illustrate that problem, and a couple of related issues

davidsminor/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,davidsminor/gaffer,boberfly/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,goddardl/gaffer,boberfly/gaffer,davidsminor/gaffer,chippey/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,cedriclaunay/gaffer,cedriclaunay/gaffer,cedriclaunay/gaffer,chippey/gaffer,hradec/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,chippey/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,goddardl/gaffer,boberfly/gaffer,goddardl/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,chippey/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,davidsminor/gaffer,lucienfostier/gaffer,goddardl/gaffer,lucienfostier/gaffer,boberfly/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,hradec/gaffer,johnhaddon/gaffer,hradec/gaffer