http: fix multipart reply

fixes compatibility with libneon ( davix and ROOT )

after second reading of http://tools.ietf.org/html/rfc2616
each Range block should not contain Content-length, but
complete requests must.

Acked-by: Gerd Behrmann
Target: master, 2.8, 2.7, 2.6
Require-book: no
Require-notes: no
(cherry picked from commit 702c68b36d369876a70a5ef8cf81dded9cf740ec)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>