'child_removed' triggers on setting listener, before child is removed

[skwny]

The AngularFireDatabase child_removed event triggers on initially adding the listener, not when children are removed.

Version info
Angular:
8.2.2

Firebase:
6.4.0

AngularFire:
5.2.1

Other (e.g. Node, browser, operating system) (if applicable):
OS: Mac 10.14.6
Browser: Chrome 76.0.3809.132

Steps to reproduce
Set a listener for 'child_added' event:

      constructor(private afdb: AngularFireDatabase) {}

      this.afdb
        .list('items)
        .valueChanges(['child_removed'])
        .subscribe((values) => {
          console.log(values);
        });

Notice that the listener will trigger on setting the listener.

Actual behavior
Listener triggers before child nodes are removed.

Expected:
Listener will not trigger until children nodes are removed.