Updates firebaseApp check in makeDocumentSnapshot

Fixes firebase#79

Author: rharter

src/providers/firestore.ts

@@ -55,7 +55,7 @@ export function makeDocumentSnapshot(
 ) {
   let firestoreService;
   let project;
-  if (has(options, 'app')) {
+  if (has(options, 'firebaseApp')) {
     firestoreService = firestore(options.firebaseApp);
     project = get(options, 'app.options.projectId');
   } else {