Return value is always parsed as json which breaks exports

[Fardenz]

All requests are parsed assuming the response is going to be always JSON type, not taking into account the exceptions in the special endpoints. One of them is the export endpoint, which can return binary responses (png, pdf...) causing the return value to be null.

https://github.com/cumulio/cumul.io-sdk-php/blob/cabb8bda4bdb4f9f308f8bf755439c8211644e16/Cumulio.php#L97