[rackspace|storage] fixed issue in ruby 1.8.7 where metadata was not being deleted

File metadata is not being removed when set to nil when using ruby 1.8.7. This bug was exposed by running the file_tests without using mocks.

After reviewing the specs we should have been setting the key names to X-Remove-Object-Meta-<keyname> when we are trying to delete keys.

http://docs.rackspace.com/files/api/v1/cf-devguide/content/Update_Object_Metadata-d1e2338.html