use protocol signaled by 'X-Forwarded-Proto' header (document_webdav)

The 'document_webdav' module has twice the line
 getattr(self.server,'proto', 'http').lower()
If OpenERP is behind a reverse SSL proxy, this causes links to be generated with the wrong protocol. To work around this issue this patch allows to override the default 'http' setting by the well established 'X-Forwarded-Proto' HTTP header.