[Important] allow sending data ONLY notifications for FCM notifications

[SebastienMelki]

Hey @appleboy ran into a little issue here need this tweak to be able send and process background notifications on android

If you want to process android notifications in the BACKGROUND, they need to be data notifications, as in not contain the "notification" key in the payload.

This little change achieves it, because omitempty on structs for the json encoding does not work on sub-structs except if it is a pointer to it and set to nil.

[appleboy]

@SebastienMelki https://cloud.drone.io/appleboy/gorush/195/1/7

Please update the code format. Try make fmt command.

[appleboy]

code format error.

[SebastienMelki]

code format error.

on it thanks

[SebastienMelki]

make fmt

done

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8929554). Click here to learn what that means.
The diff coverage is 75.86%.

@@            Coverage Diff            @@
##             master     #453   +/-   ##
=========================================
  Coverage          ?   83.05%           
=========================================
  Files             ?       20           
  Lines             ?     1393           
  Branches          ?        0           
=========================================
  Hits              ?     1157           
  Misses            ?      188           
  Partials          ?       48

Impacted Files	Coverage Δ
gorush/notification.go	100% <ø> (ø)
config/config.go	93.97% <ø> (ø)
gorush/notification_fcm.go	82.3% <ø> (ø)
gorush/server.go	85% <100%> (ø)
storage/badger/badger.go	77.01% <100%> (ø)
gorush/notification_apns.go	85.23% <72.72%> (ø)
gorush/server_normal.go	78.04% <78.04%> (ø)

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[appleboy]

@SebastienMelki Thanks.