The server may set this optional response header to change the document title after a fragment update.
The title must be encoded as a JSON string.
Without this header Unpoly will extract the
<title> from the server response.
This header is useful when you optimize your response to not render
the application layout unless targeted. When your shortened response
no longer includes a
<title>, you can instead use this HTTP header to pass the document title.
HTTP headers may only contain US-ASCII (7-bit) characters. If you have higher code points in a JSON value, you may encode those characters using Unicode escape sequences.
X-Up-Title: "Playlist browser"
Note that the quotes must be included in the JSON-encoded header value.