Loosing absolute path on assignation

Hello,

When playing with `file://`uris I found a weird behaviour related to `path` setter.

### Steps to reproduce

```
>>> x = furl('file:///a/b')
>>> x.path
Path('/a/b')
>>> x.path.isabsolute # The path here is absolute and is ok
True
>>> x.path = x.path / 'c'
>>> x.path # Once set to the path attribute of a furl object the path is not absolute anymore
Path('a/b/c')
>>> x.path.isabsolute
False
```

#### Additional info & workaround

This problem does not happen whan assigning a path as a string

```
>>> x = furl('file:///a/b')
>>> x.path = str(x.path / 'c')
>>> x.path
Path('/a/b/c')
>>> x.path.isabsolute
True
```

### Expected behaviour

The concatenation of a path segment should not change the way the path interpreted