deletes the old key from items map only if the old key is not nil (#4)

Co-authored-by: prpeh <[email protected]>

ringcache.go

@@ -71,13 +71,10 @@ func (c *RingCache) Add(key, value interface{}) (evicted bool) {
 			c.onEvict(k, c.items[k])
 			evicted = true
 		}
+		delete(c.items, k)
 	}
 
-	size := c.Len()
 	c.items[key] = value
-	if size < c.Len() {
-		delete(c.items, k)
-	}
 	c.keys[c.next] = key
 	c.next = (c.next + 1) % c.maxSize
 	return