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