client.go
func (c *Client) EnsureIndex(index *Index) error {
	err := c.CreateIndex(index)
	if err == ErrIndexExists {
		return nil
	}
	return err
}
func (c *Client) EnsureIndex(index *Index) error {
	err := c.CreateIndex(index)
	if err == ErrIndexExists {
		return nil
	}
	return err
}
// EnsureIndex creates an index on the server if it does not exist.