rxfire: objectVal bugfix

[jhuleatt]

Fixing a bug where RxFire's objectVal will return an empty object if the result of snapshot.val() is not an object. For example:

database data:

{
    "counter": 7
}

code:

fromRef(firebase.database().ref('counter')).pipe(value => console.log(value));

logs {} instead of the expected 7.

[jamesdaniels]

lgtm