We aren't yet ready to make FileSystem a public type, but I don't mind
making it _almost_ available via an internal API for those brave enough
to try that.
Also migrate all of our tests to use the in memory file system. It's simpler.
https://github.com/square/okhttp/issues/1459
I contemplated supporting them in HttpUrl, but then realized I'd
either need to document that behavior (which is weird behavior)
or return ws from the scheme (which seems arbitrary.)
Doing it in Request seems less bad: we support websockets requests.
