client.go
func (c *Client) Post(dst []byte, url string, postArgs *Args) (statusCode int, body []byte, err error) {
	return clientPostURL(dst, url, postArgs, c)
}
func (c *Client) Post(dst []byte, url string, postArgs *Args) (statusCode int, body []byte, err error) {
	return clientPostURL(dst, url, postArgs, c)
}
// Post sends POST request to the given url with the given POST arguments.
//
// The contents of dst will be replaced by the body and returned, if the dst
// is too small a new slice will be allocated.
//
// The function follows redirects. Use Do* for manually handling redirects.
//
// Empty POST body is sent if postArgs is nil.