URL: Appending to an empty file path results in an absolute path

[jrflat]

When appending path to a URL with an empty relative path, the current code does: "" + "/" + path. This is problematic because it makes the relative path absolute, so it no longer resolves against the base URL.

E.g. for the following code,

let currentDir = URL(filePath: "")
print(currentDir.path) // "/my/cwd"
let file = currentDir.appending(path: "main.swift")
print(file.path) // "/main.swift"

we should get an output of "/my/cwd/main.swift", not "/main.swift".

This PR updates URL.appending(path:) so if the current path is empty (and thus relative), we don't append a slash which would make the path absolute. That is unless we have an authority. If we have an authority, we must add a slash to separate the path from the authority, e.g. URL("http://example.com").appending(path: "path") == http://example.com/path.

Resolves #921 (thanks @omochi for filing!)

[jrflat]

@swift-ci please test

[omochi]

Thank you for fixing this!​

[jrflat]

Thanks again for bringing this to our attention! I believe this also resolves #922 (the relative path will still be empty, but we will handle it correctly afterwards). I've opened #989 to fix some other special cases of .deletingLastPathComponent() as well

[jrflat]

Updated the behavior slightly to be more compatible for clients expecting the relative path to start with "./".

[jrflat]

@swift-ci please test