fix(Firebase): Only set timestampsInSnapshots when explicitly set (#42)

As of firebase@5.8.x timestampsInSnapshots is true implicitly
#39

Furthermore: calling settings() multiple times is not supported.
If your app interacts with Firestore prior to using <FirestoreProvider>,
FirestoreProvider's settings() call will get you into trouble:

Uncaught FirebaseError: Firestore has already been started
and its settings can no longer be changed. You can only call settings()
before calling any other methods on a Firestore object

src/FirestoreProvider.js

@@ -11,17 +11,17 @@ export default class FirestoreProvider extends Component {
     useTimestampsInSnapshots: PropTypes.bool,
   };
 
-  static defaultProps = {
-    useTimestampsInSnapshots: false,
-  };
+  static defaultProps = {};
 
   constructor(props) {
     super(props);
 
     const { firebase, useTimestampsInSnapshots } = props;
-    const settings = { timestampsInSnapshots: useTimestampsInSnapshots };
     const firestore = firebase.firestore();
-    firestore.settings(settings);
+    if (typeof useTimestampsInSnapshots !== 'undefined') {
+      const settings = { timestampsInSnapshots: useTimestampsInSnapshots };
+      firestore.settings(settings);
+    }
 
     this.state = {
       firestoreDatabase: firestore,

src/__tests__/provider.firestore-settings.js

@@ -15,19 +15,27 @@ test('sets timestampsInSnapshots correctly', () => {
     </FirestoreProvider>,
   );
 
+  expect(firestore.settings).toHaveBeenCalledTimes(0);
+
+  mount(
+    <FirestoreProvider firebase={firebase} useTimestampsInSnapshots>
+      <div>Test</div>
+    </FirestoreProvider>,
+  );
+
   expect(firestore.settings).toHaveBeenCalledTimes(1);
   expect(firestore.settings).toHaveBeenCalledWith({
-    timestampsInSnapshots: false,
+    timestampsInSnapshots: true,
   });
 
   mount(
-    <FirestoreProvider firebase={firebase} useTimestampsInSnapshots>
+    <FirestoreProvider firebase={firebase} useTimestampsInSnapshots={false}>
       <div>Test</div>
     </FirestoreProvider>,
   );
 
   expect(firestore.settings).toHaveBeenCalledTimes(2);
   expect(firestore.settings).toHaveBeenCalledWith({
-    timestampsInSnapshots: true,
+    timestampsInSnapshots: false,
   });
 });