Fix `hash_hmac` arguments order

From PHP document [hash_hmac](http://sg2.php.net/manual/en/function.hash-hmac.php)

    string hash_hmac ( string $algo , string $data , string $key [, bool $raw_output = false ] )

But in Request.php

    return hash_hmac('sha256', $token->getSecret(), $this->stringToSign());

`hash_hmac` expects `data` first then `key`