Payment specs

Fixes 3 of 4 specs for `Conekticut::Client::Payment`. Remaining broken spec is due to the card tokenizing requirement on Conekta's API

__pending ideas__

* Add a `Conekticut::Client::Card` class that handles a credit/debit card data and proper validations.
* Add a `Conekticut::Client::Tokenizer` class that handles `tokenizing` a card by implementing `Conekta.js` library functionality, [available here](https://conektaapi.s3.amazonaws.com/v0.3.0/js/conekta.js)
Surrounding space missing for operator '=>'.