MINOR: http/hardening: implement hardening "reject-content-length-leading-zero"

MINOR: http/hardening: implement hardening "reject-content-length-leading-zero"

Rejects requests and responses having a Content-Length header field
whose value contains a leading zero before a digit. Some client and
server applications tend to parse this as an octal value giving a
different body size (e.g. "0300" = 192). This can be used in attacks
trying to desynchronize a client or server from the rest of the
communication, and for example for cache poisionning attacks. These may
seldom be seen on legacy applications but not on standard compoennts
found on the internet, so it is generally safe to reject them on public
facing listeners.