APIError.Details empty for error responses

[kayslay]

APIError.Details was empty because the ioutil.ReadAll was reading an empty http.Response.Body.

The body of the response was read earlier by the decodeResponse func before attempting to read it again in newAPIError.

Changed the signature of decodeResponse to accept a []byte as the second argument.

Example commit. Check changes made to paystack.go and error.go

[gray-adeyi]

Hi @kayslay try out this project https://gray-adeyi.github.io/paystack/