Ensure expired items are removed from the cache

spec/stores/memory_store_spec.cr

@@ -204,6 +204,21 @@ describe LuckyCache::MemoryStore do
         cache.read("key").should eq(nil)
       end
     end
+
+    it "evicts expired items from the cache to free memory" do
+      cache = LuckyCache::MemoryStore.new
+      cache.write("key1", expires_in: 1.minute) { "data1" }
+      cache.write("key2", expires_in: 5.minutes) { "data2" }
+      cache.size.should eq(2)
+
+      Timecop.travel(90.seconds.from_now) do
+        cache.read("key1").should eq(nil)
+        cache.size.should eq(1)
+
+        cache.read("key2").should_not be_nil
+        cache.size.should eq(1)
+      end
+    end
   end
 
   describe "#delete" do

src/lucky_cache/stores/memory_store.cr

@@ -7,10 +7,15 @@ module LuckyCache
     end
 
     # Returns the `CacheItem` or nil if the `key` is not found.
-    # If the key is found, but the item is expired, it returns nil.
+    # If the key is found, but the item is expired, remove it and return nil.
     def read(key : CacheKey) : CacheItem?
       if item = cache[key]?
-        item.expired? ? nil : item
+        if item.expired?
+          cache.delete(key)
+          nil
+        else
+          item
+        end
       end
     end
 
@@ -75,6 +80,7 @@ module LuckyCache
       end
     end
 
+    # The total number of items in the cache.
     def size : Int32
       @cache.size
     end