Fix chunked body gather-write test.

The braced array literals seem to be constructed as temporaries. The test passes in debug builds, but in optimized builds, the arrays have been overwritten by the time they are used.

c++/src/kj/compat/http-test.c++

@@ -918,7 +918,9 @@ KJ_TEST("HttpClient chunked body gather-write") {
     auto req = client->request(HttpMethod::POST, "/", HttpHeaders(table));
 
     kj::ArrayPtr<const byte> bodyParts[] = {
-      { 'f','o','o' }, { ' ' }, { 'b','a','r' }, { ' ' }, { 'b','a','z' }
+      "foo"_kj.asBytes(), " "_kj.asBytes(),
+      "bar"_kj.asBytes(), " "_kj.asBytes(),
+      "baz"_kj.asBytes()
     };
 
     req.body->write(kj::arrayPtr(bodyParts, kj::size(bodyParts))).wait(waitScope);