Fix default value description for URL

[Frizlab]

At first I noticed a crash when running the tests of this project. It was due to weird chars in my CWD.
This lead me to discover a bug where the default value description of a URL checks for the absoluteString value of itself instead of the path + whether it is a file URL!

Note: I did not add a test specifically, I don’t really know how this would be tested. Maybe create a folder with a weird name and check everything is good? Also a test with an https URL as default w/ a path being the same as current CWD?
Tell me, I’ll do them if needed.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[kylemacomber]

After a quick skim of the documentation, it looks like this might actually be a Foundation bug. FileManager.currentDirectoryPath is defined as returning a type String, but it appears possible it will return nil. It should probably be defined as returning String?.

[Frizlab]

@kylemacomber I don’t think so. Maybe it’s a separate issue, but unrelated to the one I reported.

URL(string: FileManager.default.currentDirectoryPath) is likely to fail, and is definitely not supposed to be used to create a file URL.
It might not return nil if by a lucky chance the current directory path happens to be a valid URL (e.g. a path with no space in it), but 1/ the returned URL will never be a file URL, and 2/ if the current path contains a space or any character not allowed in a URL the init will definitely return nil.

The correct way to init a file URL is URL(fileURLWithString: "/path/to whatever"), which will never return nil in addition to correctly returning a file URL.

EDIT: And of course, absoluteString is not the path of a URL. For a file URL it could yield something like file:///path/to%20whatever for instance.

[kylemacomber]

@swift-ci please test