rename (key? -> cached?) and make public

lib/review/book/cache.rb

@@ -21,19 +21,19 @@ def reset
       def fetch(key, &block)
         raise ArgumentError, 'Key should be Symbol' unless key.is_a?(Symbol)
 
-        if key?(key)
+        if cached?(key)
           read(key)
         else
           exec_block_and_save(key, &block)
         end
       end
 
-      private
-
-      def key?(key)
+      def cached?(key)
         @store.key?(key)
       end
 
+      private
+
       def read(key)
         @store[key]
       end