Fix mkdir when directory is top level

[tannewt]

No description provided.

[antonio-openroad]

Mkdir now works if directory is top level, but it crashes if the directory is in the second level

mkdir /foo -> ok
mkdir /foo/bar -> crashes

Crash message: TypeError: 'str' object does not support item assignment

[tannewt]

Thanks for testing @antonio-openroad. I'll fix it this week and update this PR.

[tannewt]

Note, I'm in the weeds of improving the test code. There are at least a few places that don't handle packet merging well. So, this may slip until next week. I'll get all of my changes pushed before I leave on next Thursday though.