Fix HEAD request handling in bup-web

HEAD requests to files did not set self.finished(). This causes problems
with user-agents that expect some type of request since it never closes
(OpenOffice/LibreOffice will notably stall opening files from a URL).

This patch stands alone from WebDAV but also fixes the problem there.

The problem can be demonstrated like this:

    bup web
    curl -I 'http://127.0.0.1:8080/some/file'

Signed-off-by: Will Rouesnel <w.rouesnel@gmail.com>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>