Fix leading slash for abspath in joinpath

[pb866]

Currently, joinpath gives two leading slashes, when an absolute path is joined to an empty URI path. This PR fixes this issue.

julia> using URIs

julia> u = URI("http://test.com")
URI("http://test.com")

julia> joinpath(u, "/abs/path")
URI("http://test.com//abs/path")

[pb866]

Is there anything that needs to be done before this can be merged? I think, the failing test has nothing to do with this fix.

An alternative fix would be to ensure non-empty URI paths, so the condition that was adjusted in this PR is not needed at all. This is also a workaround for the current situation (without tempering with the package's code), but it would be nice to have a proper resolution one way or the other.

[ararslan]

Thanks for the contribution and apologies for the delayed response. What you've implemented here seems reasonable to me and is what I would generally have expected joinpath to do in this case. My read of RFC 2396 also suggests that what you're proposing is the correct behavior and joinpath is currently (arguably) not compliant. However, this is technically breaking as some code may be relying on the current behavior. I'm in favor of merging but would want a second opinion; @quinnj?