BUG/MEDIUM: httpclient/lua: infinite appctx loop with POST

When POSTing a request with a payload, and reusing the same httpclient
lua instance, one could encounter a spinning of the httpclient appctx.

Indeed the sent counter is not reset between 2 POSTs and the condition
for sending the EOM flag is never met.

Must fixed issue #1593.

To be backported in 2.5.

src/hlua.c

@@ -7284,6 +7284,7 @@ __LJMP static int hlua_httpclient_send(lua_State *L, enum http_meth_t meth)
 		return 0;
 	}
 
+	hlua_hc->sent = 0;
 
 	hlua_hc->hc->req.url = istdup(ist(url_str));
 	hlua_hc->hc->req.meth = meth;