Before this change if the server sent us xml like this
```
<D:propstat>
<D:prop>
<g0:quota-available-bytes/>
<g0:quota-used-bytes/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
```
Rclone would read the empty XML items as containing 0
After this fix we make sure that we have a value before using it.
Bitrix Site Manager emits `<D:resourcetype><collection/></D:resourcetype>`
missing the namespace on the `collection` tag. This causes the item
to be identified as a file instead of a directory.
To work around this look at the Microsoft extension prop
`iscollection` which seems to be emitted as well.
The time format provided by webdav servers seems to vary wildly from
that specified in the RFC - rclone already parses times in 5 different
formats!
If an unparseable time is found, then fail softly logging an ERROR
(just once) but returning the epoch.
This will mean that webdav servers with bad time formats will still be
usable by rclone.
