Honor fog_credentials[:scheme] option

So that `attachment.url` can provide an HTTP url if properly configured:
```
has_attached_file :image,
  storage: :fog,
  :fog_credentials => {
    :scheme            => 'http',
    :access_key_id     => ENV['AWS_ACCESS_KEY_ID'],
    :secret_access_key => ENV['AWS_SECRET_ACCESS_KEY']
  }
```

Default is still HTTPS, indeed.