Encoding::UndefinedConversionError when downloading an image

For instance:

```
> RestClient::Request.execute(method: :get, url: "http://lorempixel.com/200/300/", raw_response: true)
Encoding::UndefinedConversionError: "\x8B" from ASCII-8BIT to UTF-8
```

I'm using `raw_response: true` so I can get a response with a tempfile instead of keeping the data in a string.