fetching files renders invalid blob

How it should be
```js
let e = await fetch('https://i.imgur.com/4AiXzf8.jpg')
// Response { type: "cors", url: "https://i.imgur.com/4AiXzf8.jpg", redirected: false, status: 200, ok: true, statusText: "OK", headers: // Headers, body: ReadableStream, bodyUsed: false }
await e.blob()
// Blob { size: 87363, type: "image/jpeg" }
```

How it is with unfetch
```js
let e = await fetch('https://i.imgur.com/4AiXzf8.jpg')
// Object { ok: true, statusText: "OK", status: 200, url: "https://i.imgur.com/4AiXzf8.jpg", text: text(), json: json(), blob: blob(), clone: a(), headers: {…} }
await e.blob()
// Blob { size: 156997, type: "" }
```
Note how the blob's size is different and the type is missing.

That can also be confirmed by using [`FileReader.readAsDataURL()`](https://developer.mozilla.org/en-US/docs/Web/API/FileReader/readAsDataURL) on the generated blob, the resulting file is invalid and cannot be opened.