Reverted XMLHttpRequest change

Apparently the fetch especification breaks on some older samsung devices, a patch was made trying to fix this by using XMLHttpRequest, however this brings other problems, including breaking compatibility with node.js, so I undid this change, and this is final, if you face any problem on some older devices, my suggestion is to handle the error as gracefully as possible or roll your own implemntation on server-side (CORS is there for a reason), and push the react-native guys to fix the issue in future versions.