`mongo_dav_provider.py` code appears to be out of date

[John T. Wodder II]

The file wsgidav/samples/mongo_dav_provider.py appears to be out of date in at least the following respects:

• This line constructs an io.StringIO from a bytes object, something that is only possible on Python 2.
• ConnectionCollection.get_member_names(), DbCollection.get_member_names(), and DocResource.get_display_name() return or can return bytes, yet I am quite sure that they should always be returning str values instead. In particular, the values returned by get_member_names() are only ever used by being passed to get_member(), yet the classes' get_member() methods pass their arguments to join_uri(), which only works on strs.

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