get aux_token on auth

It will be nice [to get aux_token](https://tech.yandex.ru/money/doc/dg/reference/obtain-token-aux-docpage/)
**Notice:**
Test assumes, that before running tests you 

1) Have ACCESS_TOKEN
2) Obtained it with scope as minimum as "account-info"
> Например, у вас есть access_token со scope = account-info payment-p2p. Вы можете сделать запрос дополнительного токена со scope = account-info. 

Of'course VCR saved my request (for usual testing demands above is not needed), but for development purposes it is important notice.
`end` at 43, 0 is not aligned with `it "should get auxiliary token" do` at 37, 2