joinpath strips out path information when a parameter contains leading slash

[omus]

julia> joinpath("s3://", "a", "b")
"s3://a/b"

julia> joinpath("s3://", "a", "/b")
"/b"

[omus]

Whoops, I didn't realize this was a Base.Filesystem behaviour

[rofinn]

I actually think we might still want to drop this behaviour since it's unintuitive (IMHO) and introduces some weird type instabilities that make it even harder to reason about what joinpath is doing.