Fix renderPager undefined override

[mmtootmm]

Motivation

react-native-tab-view provides some pager alternatives like PagerExperimental but we can't use it with react-navigation-tabs because it always overrides renderPager prop with undefined.

Test plan

When useNativeDriver={true}, built-in MaterialTopTabBar not working because color style prop not supported by native animation.

createMaterialTopTabNavigator({
  ...
}, {
  tabBarComponent: CustomTabBarComponent,
  renderPager: props => <PagerExperimental {...props} {GestureHandler} userNativeDriver={true} />
})