Set Connection: close before calling vcl_pipe

It makes sense to default to setting Connection: close for vcl_pipe.
Do this before calling vcl_pipe so it's overrideable.

Fixes #1455

bin/varnishd/cache/cache_req_fsm.c

@@ -579,6 +579,7 @@ cnt_pipe(struct worker *wrk, struct req *req)
 	http_FilterReq(bo->bereq, req->http, 0);	// XXX: 0 ?
 	http_PrintfHeader(bo->bereq,
 	    "X-Varnish: %u", req->vsl->wid & VSL_IDENTMASK);
+	http_SetHeader(bo->bereq, "Connection: close");
 
 	VCL_pipe_method(req->vcl, wrk, req, bo, req->http->ws);