[iOS GoogleMap] Fix animateCamera (react-native-maps#2608)

Fixed a bug that
zoom and heading will always be 0 if they are not included in the argument of animateCamera

Author: unosk

lib/ios/AirGoogleMaps/RCTConvert+GMSMapViewType.m

@@ -42,8 +42,8 @@ + (GMSCameraPosition*)GMSCameraPositionWithDefaults:(id)json existingCamera:(GMS
         viewingAngle = existingCamera.viewingAngle;
         latitude = existingCamera.target.latitude;
         longitude = existingCamera.target.longitude;
-        zoom = 0;
-        bearing = 0;
+        zoom = existingCamera.zoom;
+        bearing = existingCamera.bearing;
     }
 
     if (json[@"center"]) {