Delete expired items before setting

Check if an item exists and it has expired before setting so that
onEvicted is actually called.

Fixes #48.

cache.go

@@ -58,6 +58,11 @@ func (c *cache) Set(k string, x interface{}, d time.Duration) {
 		e = time.Now().Add(d).UnixNano()
 	}
 	c.mu.Lock()
+	if i, ok := c.items[k]; ok && i.Expired() {
+		// delete if expired so .onEvicted is called
+		c.delete(k)
+	}
+
 	c.items[k] = Item{
 		Object:     x,
 		Expiration: e,