fix digitalocean v2 api content-type when using json

In case of using json we need to set content-type header.

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
This is already handled above by the err check, as `client.Do` returns an error if non-200:

> An error is returned if caused by client policy (such as CheckRedirect), or if there was an HTTP protocol error. A non-2xx response doesn't cause an error. (http://golang.org/pkg/net/http/#Client.Do)

So, you can remove this bit I think.