SFTPFS: improper isfile/isdir() results for URL pointing to a file

Point SFTPFS to a file instead of a directory returns improper results for isdir()

```
from fs.sftpfs import SFTPFS

handle = SFTPFS('localhost', '/tmp')
print handle.isdir('.') # True
print handle.isfile('.') # False

handle = SFTPFS('localhost', '/tmp/fswrapper.py')
print handle.isdir('.') # True !!!!!!
print handle.isfile('.') # True
```
