Error callback has nested error object stringified

[mirkods]

https://github.com/Stamplay/stamplay-product-development/issues/48

The error cames from this line:
https://github.com/Stamplay/stamplay-js-sdk/blob/master/src/promise.js#L39

Probably a good fix is don't wrap the server data in an object in case of error and if the content-type is application/json parse the response and returns it has arrived from the server.