Fix trailing slash behavior

[ap--]

Closes #483

Trailing slash behavior is now identical for:

UPath("proto://path/key/")
UPath("proto://path/", "key/")
UPath("proto://path/").joinpath("key/")

If trailing slashes are kept or not depends on the parser. Currently only parser that does is http/https.

[ap--]

mac builds failing due to runner issue: actions/runner-images#13341