The server may set this response header to accept the targeted overlay
in response to a fragment update.
Upon seeing the header, Unpoly will cancel the fragment update and accept the layer instead.
If the root layer is targeted, the header is ignored and the fragment is updated with
the response's HTML content.
The header value is the acceptance value serialized as a JSON object.
To accept an overlay without value, set the header value to the string null.
Example
The response below will accept the targeted overlay with the value {user_id: 1012 }:
Content-Type: text/html
X-Up-Accept-Layer: {"user_id": 1012}
<html>
...
</html>
Rendering content
The response may contain text/html content. If the root layer is targeted,
the X-Up-Accept-Layer header is ignored and the fragment is updated with
the response's HTML content.
If you know that an overlay will be closed don't want to render HTML,
have the server change the render target to :none:
Content-Type: text/html
X-Up-Accept-Layer: {"user_id": 1012}
X-Up-Target: :none
