http: fix multipart response size

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: no
(cherry picked from commit 5c63d0e26a408eacd1c5d8d8398e47fd3eaad045)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>