Adding support for 'Prefer: wait=0' header with monitor.

Motivation:
When a device request to monitor it can also request that the server
send out any non delivered messages straight away by sending the Prefer
header.
This is described in the 'Monitor and Receiving Push Message' section
[1] of the specification.

[1] https://tools.ietf.org/html/draft-thomson-webpush-http2-01#section-6