[Improvement] Add this line to code for fix actionable notifications

[roman-rr]

Hey @dpa99c
I don't wanna create new pull request (it will override some my custom local changes).
But please, just add this line into AppDelegate+FirebasePlugin.m line number 76

// Delegate notification center here (prevent missing notifications)
[UNUserNotificationCenter currentNotificationCenter].delegate = self;

It is required by apple docs

Important
You must assign your delegate object to the UNUserNotificationCenter object before your app finishes launching. For example, in an iOS app, you must assign it in the application:willFinishLaunchingWithOptions: or application:didFinishLaunchingWithOptions: method of your app delegate. Assigning a delegate after these methods are called might cause you to miss incoming notifications.

In my case it solve problem when app was terminated by user, and then first actionable notification doesn't trigger function in background. Only first, because it is call application:didFinishLaunchingWithOptions:, and notifications there was not ready.

[spennyf]

Hey, thanks for the great work on this plugin, any chance this will be merged in soon?