avoid re-setting Accept-Encoding for ESI

we should not emit ReqUnset / ReqHeader if A-E is already correct

reflects 6dbfff12dca895534b090ef9283e75f85659fcfa in varnish-cache

src/vdp_pesi.c

@@ -355,13 +355,14 @@ vped_include(struct req *preq, const char *src, const char *host,
 	/* Don't allow Range */
 	http_Unset(req->http, H_Range);
 
-	/* Set Accept-Encoding according to what we want */
-	http_Unset(req->http, H_Accept_Encoding);
-
 	CHECK_OBJ_NOTNULL(node->parent, NODE_MAGIC);
 	assert(node->parent->type == T_NEXUS);
+
+	/* Set Accept-Encoding according to what we want */
 	if (node->parent->nexus.gzip.is)
 		http_ForceHeader(req->http, H_Accept_Encoding, "gzip");
+	else
+		http_Unset(req->http, H_Accept_Encoding);
 
 	/* Client content already taken care of */
 	http_Unset(req->http, H_Content_Length);