MINOR: Do not forward the header "Expect: 100-continue" when the option http-buffer...

MINOR: Do not forward the header "Expect: 100-continue" when the option http-buffer-request is set

When the option "http-buffer-request" is set, HAProxy send itself the
"HTTP/1.1 100 Continue" response in order to retrieve the post content.
When HAProxy forward the request, it send the body directly after the
headers. The header "Expect: 100-continue" was sent with the headers.
This header is useless because the body will be sent in all cases, and
the server reponse is not removed by haproxy.

This patch removes the header "Expect: 100-continue" if HAProxy sent it
itself.