Return response from SnappPay::eligible

[amirzpr]

Currently, all SnappPay endpoints except /oauth/token have a specific JSON structure in their response as below:

{
    "response": {},
    "successful": true
}

The SnappPay driver implemented in this project has followed this structure and is checking the successful boolean property first (throwing an exception if it's false), then returning the response property as result in all public methods. But this convention has been ignored in the eligible method. The successful property check is absent, and it's returning the entire json body instead of the response property.

I submitted this PR to follow the same convention for returning results in all public methods. I am aware that this change may introduce a breaking change, so please let me know how you handle these kind of inconsistencies.