Some server have HEAD method disabled or not implemented.
So we can use GET with no data, retrieving only the header info.

The implementing code comes from LO WebDAV serf implementation.

Change-Id: I3b3d81f5a07232c4f1647888685e820939bc978c
Reviewed-on: https://gerrit.libreoffice.org/28982Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Giuseppe Castagno <giuseppe.castagno@acca-esse.eu>