client.go
func (c *Client) EnsureField(field *Field) error {
	err := c.CreateField(field)
	if err == ErrFieldExists {
		return nil
	}
	return err
}
func (c *Client) EnsureField(field *Field) error {
	err := c.CreateField(field)
	if err == ErrFieldExists {
		return nil
	}
	return err
}
// EnsureField creates a field on the server if it doesn't exists.