Add SetDefault() for setting with the default expiration

cache.go

@@ -81,6 +81,12 @@ func (c *cache) set(k string, x interface{}, d time.Duration) {
 	}
 }
 
+// Add an item to the cache, replacing any existing item, using the default
+// expiration.
+func (c *cache) SetDefault(k string, x interface{}) {
+	c.Set(k, x, DefaultExpiration)
+}
+
 // Add an item to the cache only if an item doesn't already exist for the given
 // key, or if the existing item has expired. Returns an error otherwise.
 func (c *cache) Add(k string, x interface{}, d time.Duration) error {