Fix cases when we do not want to send a 100 Continue

- when we already started reading the body
- for HTTP Versions less than 1.1

bin/varnishd/http1/cache_http1_fsm.c

@@ -328,6 +328,7 @@ http1_dissect(struct worker *wrk, struct req *req)
 			wrk->stats->client_req_417++;
 			return (-1);
 		}
+		if (req->http->protover >= 11 && req->htc->pipeline_b == NULL)
 			req->want100cont = 1;
 		http_Unset(req->http, H_Expect);
 	}

bin/varnishtest/tests/c00018.vtc

-varnishtest "Check Expect headers"
+varnishtest "Check Expect headers / 100 Continue"
 
 server s1 {
 	rxreq
 	txresp
+	rxreq
+	txresp
 } -start
 
 varnish v1 -vcl+backend { } -start
 
 client c1 {
-	txreq -url "/" -req POST -hdr "Expect: 100-continue " -body "foo"
+	txreq -url "/" -req POST -hdr "Expect: 100-continue " \
+		-hdr "Content-Length: 20"
 	rxresp
 	expect resp.status == 100
+	send "01234567890123456789"
+	rxresp
+	expect resp.status == 200
+
+	txreq -url "/" -req POST -hdr "Expect: 100-continue " -body "foo"
 	rxresp
 	expect resp.status == 200
+
 	txreq -url "/" -req POST -hdr "Expect: 101-continue" -body "foo"
 	rxresp
 	expect resp.status == 417