fetch method should not return void but undefined instead

PR-URL: #303
Credit: @Thenkei
Close: #303
Reviewed-by: @isaacs

EDIT(@isaacs): updated to preserve `fetchMethod`'s ability to return
`void`.

BREAKING CHANGE: change to public type signature

src/index.ts

@@ -500,7 +500,7 @@ export namespace LRUCache {
     key: K,
     staleValue: V | undefined,
     options: FetcherOptions<K, V, FC>
-  ) => Promise<V | void | undefined> | V | void | undefined
+  ) => Promise<V | undefined | void> | V | undefined | void
 
   /**
    * Options which may be passed to the {@link LRUCache} constructor.