// Token uses client credentials to retreive a token.
// The HTTP client to use is derived from the context.
// If nil, http.DefaultClient is used.
func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type": {"client_credentials"},
		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}
// Token refreshes the token by using a new client credentials request.
// tokens received this way do not include a refresh token
func (c *tokenSource) Token() (*oauth2.Token, error) {
	return retrieveToken(c.ctx, c.conf, url.Values{
		"grant_type": {"client_credentials"},
		"scope":      internal.CondVal(strings.Join(c.conf.Scopes, " ")),
	})
}