This request header contains the targeted layer's context, serialized as JSON.
The user may choose to not send this header by configuring
up.network.config.requestMetaKeys.
Example
X-Up-Context: { "lives": 3 }
Updating context from the server
The server may update the layer context by sending a X-Up-Context response header with
changed key/value pairs:
Content-Type: text/html
X-Up-Context: { "lives": 2 }
<html>
...
</html>
Upon seeing the response header, Unpoly will assign the server-provided context object to
the layer's context object, adding or replacing keys as needed.
Client-side context keys not mentioned in the response will remain unchanged.
There is no explicit protocol to remove keys from the context, but the server may send a key
with a null value to effectively remove a key.
The frontend will use the server-provided context upates for both successful (HTTP status 200 OK)
and failed (status 4xx or 5xx) responses. If no X-Up-Context response header is set,
the updating layer's context will not be changed.
It is recommended that the server only places changed key/value pairs into the X-Up-Context
response header, and not echo the entire context object. Otherwise any client-side changes made while
the request was in flight will get overridden by the server-provided context.
