Incomplete header Content-Type, add charset

[dmkir]

Describe the bug
Non-ascii filenames are shown incorrectly in some webdav clients.

To Reproduce
Steps to reproduce the behavior:

1. Browse any webdav-folder with non-ascii filename with the client https://play.google.com/store/apps/details?id=com.ghisler.tcplugins.WebDAV&hl=ru&gl=US
2. See borjomi in place of non-ascii filename

Expected behavior
Make non-ascii filename to be shown correctly.

Additional context
Please, change file util.py, https://github.com/mar10/wsgidav/blob/master/wsgidav/util.py
line (aprox. 1236)
("Content-Type", "application/xml")
to line
("Content-Type", 'application/xml; charset="utf-8"')