prerender: Wait for the loading state to complete, fix 404

Prior to this fix, `render()` preemptively considered the app rendering to
be final, before completing the transition out of the loading state entirely.

This resulted in notFound pages not being identified as such, returning
them with a 200 OK status code instead of a 404 Not Found.

client/src/run-server.js

@@ -46,7 +46,7 @@ export default function render(pathname, args='', body, locals={}, cb) {
 
     lastState = S
 
-    if (S.loading || !S.isReady) {
+    if (S.view == 'loading' || S.loading > 0) {
       if (!seenLoading) {
         seenLoading = true
         clearTimeout(timeout)