fix_storage() with a root_path enters infinite loop if root_path ends with '/'

[Mark-Hetherington]

Within _gcsfs.py in the fix_storage process if the root_path ends with a slash, there is an infinite loop. This is because the dirname function will return each parent directory without the trailing slash.

I think this can be resolved easily, by a safety check for a trailing slash on the root path when the file system is created.