coinbase.go
func (c Client) GetExchangeRates() (map[string]string, error) {
	holder := map[string]string{}
	if err := c.Get("currencies/exchange_rates", nil, &holder); err != nil {
		return nil, err
	}
	return holder, nil
}
func (c Client) GetExchangeRates() (map[string]string, error) {
	holder := map[string]string{}
	if err := c.Get("currencies/exchange_rates", nil, &holder); err != nil {
		return nil, err
	}
	return holder, nil
}
// GetExchangeRates gets the current exchange rates