cached fetcher delete key

lib/fetch.js

@@ -26,6 +26,10 @@ class LRUCache {
     return value
   }
 
+  delete (key) {
+    this.cache.delete(key)
+  }
+
   set (key, value) {
     if (this.cache.has(key)) this.cache.delete(key)
     else if (this.cache.size >= this.maxSize) {