[rackspace|storage] patch #delete_multiple_objects for Swift v1.8

This commit addresses an issue where the Content-Type header is not
being set correctly in the response. This has been fixed in Swift v1.9.
openstack/swift@ad24cde120efe73fe5a9b866f74c7d3c766aa4cb

The tests have also been updated so they don't rely on changes in Swift v1.9
that affect the response body when errors are reported.
openstack/swift@6768d5b4be3e3922d8077b6c603586c1b7df2d07