Updates `firebaseApp` check in `makeDocumentSnapshot`

[rharter]

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Fixes #79

Pretty self-explainatory, the optional property check is checking for a property called app (like from RDB), but the firestore property is called firestoreApp.

The existing work-around when you want to create a snapshot in your own app is to add both properties and cast:

makeDocumentSnapshot(data, `/path/to/data`, {firebaseApp: app, app: app} as DocumentSnapshotOptions)