codec.go
func (reg *Registry) Unregister(mimetype string) {
	defer reg.mutex.Unlock()
	reg.mutex.Lock()

	delete(reg.codecs, mimetype)
}
func (reg *Registry) Unregister(mimetype string) {
	defer reg.mutex.Unlock()
	reg.mutex.Lock()

	delete(reg.codecs, mimetype)
}
// Unregister removes the codec for a mimetype from r.