dcap: fix interaction with Spacemanager

we have to set subject in message to pool as well, as
space manager processes PoolAcceptFileMessage as well.

Ticket: #8456
Acked-by: Dmitry Litvintsev
Target: master, 2.10
Require-book: no
Require-notes: no
(cherry picked from commit 22230c4e1b851891e6f874e00f526d8c2c9bf752)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>