Rootless file URIs do not round trip

[RFC-3986](https://tools.ietf.org/html/rfc3986#section-3.3) talks about rootless path URIs.
I interpret this to mean that a URI of file:relative/path.ext is a perfectly reasonably URI indicating a relative path. Unfortunately this does not round trip:

```python
>>> from furl import furl
>>> f = furl("file:relative/path.ext")
>>> f
furl('file:///relative/path.ext')
>>> f.path
Path('relative/path.ext')
>>> f2 = furl("file:///relative/path.ext")
>>> f2.path
Path('/relative/path.ext')
>>> f2
furl('file:///relative/path.ext')
```
The URI is turned from a relative path to an absolute path. In the first instance internally the object knows that it is a relative path but the stringification of the URL turns it into an absolute path.