client.mkdir: add trailing slash to the url

[skshetry]

It's a bit ugly though, but will do for now until we can work on #3.

[codecov-commenter]

Codecov Report

Merging #58 (e43c5dc) into main (d3c4493) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #58   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         3299      3311   +12     
  Branches       190       191    +1     
=========================================
+ Hits          3299      3311   +12     

Impacted Files	Coverage Δ
src/webdav4/client.py	100.00% <100.00%> (ø)
src/webdav4/urls.py	100.00% <100.00%> (ø)
tests/test_client.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d3c4493...e43c5dc. Read the comment docs.