Fixing issue #542 (#888)

Issue #542 caused an issue where setting the `tabBarButtonColor` would override the `tabBarSelectedButtonColor`. This fixes that.

ios/RCCTabBarController.m

     }
     UIImage *iconImageSelected = nil;
     id selectedIcon = tabItemLayout[@"props"][@"selectedIcon"];
-    if (selectedIcon) iconImageSelected = [RCTConvert UIImage:selectedIcon];
+    if (selectedIcon) {
+      iconImageSelected = [RCTConvert UIImage:selectedIcon];
+    } else {
+      iconImageSelected = [RCTConvert UIImage:icon];
+    }
 
     viewController.tabBarItem = [[UITabBarItem alloc] initWithTitle:title image:iconImage tag:0];
     viewController.tabBarItem.accessibilityIdentifier = tabItemLayout[@"props"][@"testID"];