Handle partial GET requests to /partial_content.txt

1. For the reason cited (avoiding IO in tests when not necessary) in my just-now reply to another of your comments: https://github.com/bspatafora/java_server/pull/3#discussion_r18621612

2/3. I think I just shouldn't have written the test in the first place: I'm just testing that Java works, since `fileNames` just wraps a method built into Java's `File` class.

But maybe more what you're getting at is making sure that details like filesystem IO are as isolated as they can be, so the real logic can be tested apart from those details. This is why I pulled the real logic into Stream, which, while it still involves IO, is more cheaply/easily tested, and less detail-specific (take some subclass of InputStream, which could be from a byte array or file or socket or other things, versus the filesystem-specific operations in my FileSystem class).