Fix applying merged backButton options

The backButton options are always applied. Even when one of its values has changed in mergeOptions.
This commit adds a function to apply the backButton, and should probably replace the existing rnn_setBackButtonIcon - as that function does more than just set the icon, it sets all options.

lib/ios/RNNBackButtonOptions.h

 @property (nonatomic, strong) Bool* showTitle;
 @property (nonatomic, strong) Bool* visible;
 
+- (BOOL)hasValue;
+
 @end

lib/ios/RNNBackButtonOptions.m

 	return self;
 }
 
+- (BOOL)hasValue {
+	return self.icon.hasValue ||
+			self.showTitle.hasValue ||
+			self.color.hasValue ||
+			self.title.hasValue;
+}
+
+
 @end

lib/ios/RNNViewControllerPresenter.m

 		[_customTitleView removeFromSuperview];
 		_customTitleView = nil;
 	}
-	
-	if (newOptions.topBar.backButton.icon.hasValue || newOptions.topBar.backButton.showTitle.hasValue || newOptions.topBar.backButton.color.hasValue || newOptions.topBar.backButton.title.hasValue) {
-		[viewController rnn_setBackButtonIcon:[newOptions.topBar.backButton.icon getWithDefaultValue:nil] withColor:[newOptions.topBar.backButton.color getWithDefaultValue:nil] title:[newOptions.topBar.backButton.showTitle getWithDefaultValue:YES] ? [newOptions.topBar.backButton.title getWithDefaultValue:nil] : @""];
+
+	if (newOptions.topBar.backButton.hasValue) {
+		UIViewController *lastViewControllerInStack = viewController.navigationController.viewControllers.count > 1 ? viewController.navigationController.viewControllers[viewController.navigationController.viewControllers.count - 2] : viewController.navigationController.topViewController;
+	    RNNNavigationOptions * resolvedOptions	= (RNNNavigationOptions *) [[currentOptions overrideOptions:newOptions] withDefault:[self defaultOptions]];
+		[lastViewControllerInStack applyBackButton:resolvedOptions.topBar.backButton];
 	}
 }
 

lib/ios/UIViewController+RNNOptions.h

 
 @class RNNBottomTabOptions;
 @class RNNNavigationOptions;
+@class RNNBackButtonOptions;
 
 @interface UIViewController (RNNOptions)
 
 
 - (void)rnn_setBackButtonIcon:(UIImage *)icon withColor:(UIColor *)color title:(NSString *)title;
 
+- (void)applyBackButton:(RNNBackButtonOptions *)backButton;
+
 - (BOOL)isModal;
 
 @end

lib/ios/UIViewController+RNNOptions.m

 #import "UIImage+tint.h"
 #import "RNNBottomTabOptions.h"
 #import "RNNNavigationOptions.h"
+#import "RNNBackButtonOptions.h"
 
 #define kStatusBarAnimationDuration 0.35
 const NSInteger BLUR_STATUS_TAG = 78264801;
 		backItem.image = color
 		? [[icon withTintColor:color] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal]
 		: icon;
-		
+
 		[self.navigationController.navigationBar setBackIndicatorImage:[UIImage new]];
 		[self.navigationController.navigationBar setBackIndicatorTransitionMaskImage:[UIImage new]];
 	}
 	
-	UIViewController *lastViewControllerInStack = self.navigationController.viewControllers.count > 1 ? [self.navigationController.viewControllers objectAtIndex:self.navigationController.viewControllers.count-2] : self.navigationController.topViewController;
+	UIViewController *lastViewControllerInStack = self.navigationController.viewControllers.count > 1 ? self.navigationController.viewControllers[self.navigationController.viewControllers.count - 2] : self.navigationController.topViewController;
 	
 	backItem.title = title ? title : lastViewControllerInStack.navigationItem.title;
 	backItem.tintColor = color;
 	lastViewControllerInStack.navigationItem.backBarButtonItem = backItem;
 }
 
+- (void)applyBackButton:(RNNBackButtonOptions *)backButton {
+	UIBarButtonItem *backItem = [UIBarButtonItem new];
+	if (backButton.icon.hasValue) {
+		UIColor *color = [backButton.color getWithDefaultValue:nil];
+		backItem.image = color ?
+				[[backButton.icon.get withTintColor:color] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal] :
+				backButton.icon.get;
+
+		[self.navigationController.navigationBar setBackIndicatorImage:[UIImage new]];
+        [self.navigationController.navigationBar setBackIndicatorTransitionMaskImage:[UIImage new]];
+	}
+
+	if ([backButton.showTitle getWithDefaultValue:YES]) backItem.title = [backButton.title getWithDefaultValue:nil];
+	if (backButton.color.hasValue) backItem.tintColor = [backButton.color get];
+	self.navigationItem.backBarButtonItem = backItem;
+}
+
 @end