Minor code cleanup

I simplified a few if statements and ran `go fmt` on the files that I modified. 
I appreciate the recursive attempt but this is still broken :P. If Scheme == "" it will return here and never hit line 53 onward. 

```
switch u.Scheme {
    case "http", "https":
         return fetchURL(img, ds)
    case "":
         ep, err := discovery.DiscoverEndpoints 
         ...
         return fetchImage(img, ds)
    default:
         return "", fmt.Errorf("rkt only supports ...")
}
```