Merge pull request #80 from Lodin/fix/ssr-2

SSR: Use real method instead of a string

Author: Lodin

src/Tree.tsx

@@ -286,7 +286,7 @@ const generateNewTree = <
   let tempRecord: NodeRecord<TNodePublicState> | null = rootRecord;
 
   const useIdleCallback =
-    typeof 'requestIdleCallback' !== 'undefined' &&
+    typeof requestIdleCallback !== 'undefined' &&
     placeholder !== undefined &&
     // If placeholder is set to null and this is the first build, idle callback
     // won't be used. It is necessary for trees with async data which can be