delay_while_idle deprecation

[juliangut]

While migrating Tify, my library to unify APNS and GCM (now FCM) services, I've noticed on Firebase documentation that delay_while_idle parameter is deprecated for FCM from Nov 15th 2016

So IMO either it should be ignored or completely removed

In order to reduce BC issues I've decided to keep the parameter (marked as deprecated) at message abstraction level BUT at the same time it's completely ignore when creating the actual FCM message