…as a target of _top

Given an HTML structure like this:

```html
<turbo-frame id="frame" target="_top">
  <a href="/src/tests/fixtures/prefetched.html">Hover to prefetch me</a>
</turbo-frame>
```

When the user hovers over the link, the turbo-frame header should not be sent
in a prefetch request because the wrapping turbo-frame has a target of `_top`.

If we don't respect this turbo-rails sees the turbo-frame header and renders
a turbo-frame response with a minimal layout that doesn't include any assets
in the head. When turbo processes the response it may find a missmatch in
tracked assets and cause a reload.