BUG/MEDIUM: httpclient: Don't set EOM flag on an empty HTX message

BUG/MEDIUM: httpclient: Don't set EOM flag on an empty HTX message

In the HTTP client, when the request body is streamed, at the end of the
payload, we must be sure to not set the EOM flag on an empty message.
Otherwise, because there is no data, the buffer is reset to be released and
the flag is lost. Thus, the HTTP client is never notified of the end of
payload for the request and the applet is blocked. If the HTTP client is
instanciated from a Lua script, it is even worse because we fall into a
wakeup loop between the lua script and the HTTP client applet. At the end,
HAProxy is killed because of the watchdog.

This patch should fix the issue #1898. It must be backported to 2.6.