dirmngr: fix handling of HTTPS redirections during HKP

* dirmngr/ks-engine-hkp.c (send_request): Reinitialize HTTP session when
following a HTTP redirection.

inspired by patch from Damien Goutte-Gattat <[email protected]>

GnuPG-Bug_id: 4566

• Signed-off-by: Daniel Kahn Gillmor <[email protected]>