Request: Webdav should support digest auth method (lp:#1439286)

[Original report](https://bugs.launchpad.net/bugs/1439286) created by **jcard (joao-fs-cardoso)**
```
When using duplicity 0.6.24 (or 0.6.25) under python 2.7.8 on openSUSE-13.2
with lighttpd-1.4.35 as a webdav server on a NAS I get the error:

PASSPHRASE=xxx FTP_PASSWORD=yyy duplicity --gpg-options="-z=0" -v9
Music/Miles\ Davis/ webdav://jcard@dns-325:8080/webdav

Using archive dir:
/home/jcard/.cache/duplicity/7e46105d7678fc1c7f37bccec3a33579
Using backup name: 7e46105d7678fc1c7f37bccec3a33579
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.dpbxbackend Succeeded
Import of duplicity.backends.ftpbackend Succeeded
Import of duplicity.backends.ftpsbackend Succeeded
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.sshbackend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Import of duplicity.backends.~par2wrapperbackend Succeeded
Using WebDAV protocol http
Using WebDAV host dns-325 port 8080
Using WebDAV directory /webdav/
Main action: inc
================================================================================
duplicity 0.6.24 (May 09, 2014)
Args: /usr/bin/duplicity --gpg-options=-z=0 -v9 Music/Miles Davis/
webdav://jcard@dns-325:8080/webdav
Linux silver 3.16.7-7-desktop #1 SMP PREEMPT Wed Dec 17 18:00:44 UTC 2014
(762f27a) x86_64 x86_64
/usr/bin/python 2.7.8 (default, Sep 30 2014, 15:34:38) [GCC]
================================================================================
Using temporary directory /tmp/duplicity-U9FkzT-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-U9FkzT-
tempdir/mkstemp-9AspmZ-1
Temp has 9271398400 available, backup will use approx 34078720.
Listing directory /webdav/ on WebDAV server
WebDAV create connection on 'dns-325' (retry 1)
WebDAV PROPFIND /webdav/ request with headers: {'Connection': 'keep-alive',
'Depth': '1'}
WebDAV data length: 0
WebDAV response status 401 with reason 'Unauthorized'.
WebDAV retry request with authentification headers.
WebDAV PROPFIND /webdav/ request2 with headers: {'Connection': 'keep-
alive', 'Depth': '1', 'Authorization': 'Digest None'}
WebDAV data length: 0
WebDAV response2 status 400 with reason 'Bad Request'.
Attempt 1 failed. BackendException: Bad status code 400 reason Bad Request.
Backtrace of previous error: Traceback (innermost last):
  File "/usr/lib64/python2.7/site-packages/duplicity/backend.py", line 344,
in _retry_fatal
    return fn(self, *args)
  File "/usr/lib64/python2.7/site-
packages/duplicity/backends/webdavbackend.py", line 300, in _list
    raise e
 BackendException: Bad status code 400 reason Bad Request.

The lighttpd error log shows

2015-04-01 16:13:23: (http_auth.c.1038) digest: missing field

The same error appears even when using webdav://user:password@host:port
instead of defining the FTP_PASSWORD variable.

If however I use '--gio dav://' it succeeds without errors on the same
server.

The same webdav:// error happens for 0.6.25 on a NAS with uclibc and a
cross compiled python-2.7.2 and extensions, but can't try the --gio option,
as I think that it makes no sense to have the gio backend on such platform.
All other backends (ftp/sftp/ftps/ssh/scp/rsync/gdocs/dpbx/s3) works OK on
this platform.
Probably unrelated -- for file: and s3: duplicity has to run with --no-
encryption, or a "File ....difftar.gpg was corrupted during upload." error
appears, although I can gpg decrypt and untar it.

The final purpose is to provide a duplicity package for the NAS running
Alt-F, a "free alternative firmware for the DLink
DNS-320/320L/321/323/325", https://sourceforge.net/projects/alt-f

Thanks
```

Story points: 6.0