The server may set this optional response header to change the HTTP method after a fragment update.
Without this header Unpoly will use the requested method.
Signaling the HTTP method after a redirect
Without an X-Up-Method header, Unpoly will assume a GET method after a redirect.
This assumption is correct for redirects with status codes 301, 302 and 303,
and incorrect for the less common codes 307 and 308.
Example
X-Up-Location: /current-url
X-Up-Method: GET
