added a unit test for headObject
added a unit test for headObject

chanchett/ds3_python_sdk,chanchett/ds3_python_sdk,SpectraLogic/ds3_python_sdk,SpectraLogic/ds3_python_sdk,chanchett/ds3_python_sdk_,chanchett/ds3_python_sdk,SpectraLogic/ds3_python_sdk