The list of HTTP headers to be used in the HTTP request for sending Google Cloud
Messaging (GCM) notifications.
- <x-sup-gcm-collapsekey> – An arbitrary string (such as "Updates Available") that is
used to collapse a group of like messages when the device is offline, so
that only the last message gets sent to the client.
- <x-sup-gcm-data> – Payload data, expressed as parameters prefixed with data and
suffixed as the key.
- <x-sup-gcm-delaywhileidle> – Optional. Requests that messages not be delivered until the
device becomes active Represented as 1 for true, or any other value for false. The default value
is false.
- <x-sup-gcm-timetolive> – Specifies how long (in seconds) the message should be kept
on GCM storage if the device is offline.