Don't delete changed items on original expiry

Use compare and delete to ensure item has not changed before deleting it

Fixes #1

cache.go

@@ -116,7 +116,7 @@ func (c *Cache[K, V]) cleaner() {
 
 			// remove all expired entries
 			for c.chain != nil && c.chain.HasExpired() {
-				c.items.Delete(c.chain.data)
+				c.items.CompareAndDelete(c.chain.data, c.chain.item)
 				c.chain = c.chain.next
 			}
 		}

cache_test.go

@@ -132,3 +132,23 @@ func TestCache_Cleaner(t *testing.T) {
 	c.sched.Wait()
 	assert.Nil(t, c.chain)
 }
+
+func TestCache_UpdateExpiry(t *testing.T) {
+	timeNow = func() time.Time { return time.Now() }
+
+	c := New[string, string]()
+	c.Set("a", "b", time.Now().Add(2*time.Second))
+
+	time.Sleep(time.Second)
+	get, b := c.Get("a")
+	assert.True(t, b)
+	assert.Equal(t, "b", get)
+
+	c.Set("a", "b", time.Now().Add(5*time.Second))
+
+	// after expiry of the first set call
+	time.Sleep(2 * time.Second)
+	get, b = c.Get("a")
+	assert.True(t, b)
+	assert.Equal(t, "b", get)
+}