don't use ioutil.ReadAll

Hi,

this rewrites the way the contents are read from the `resp.Body`. With `ioutil.ReadAll` you essentially throw the whole responses in memory and than consume it with `encoding/json`. With `json.NewDecoder`, the responses is consumed directly from the response so you don't have to download and allocate the whole thing in ram.

Plus: you ignored the error from `ReadAll` before.