_setElementsAlpha didn't change backItem arrow alpha

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If the navigation bar have a back button item it didn't change alpha value and is always displayed in statusbar

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An easy way to solve it and simplify the method:

From:
`- (void)lt_setElementsAlpha:(CGFloat)alpha
{
[[self valueForKey:@"_leftViews"] enumerateObjectsUsingBlock:^(UIView *view, NSUInteger i, BOOL *stop) {
view.alpha = alpha;
}];

[[self valueForKey:@"_rightViews"] enumerateObjectsUsingBlock:^(UIView *view, NSUInteger i, BOOL *stop) {
    view.alpha = alpha;
}];

UIView *titleView = [self valueForKey:@"_titleView"];
titleView.alpha = alpha;

// when viewController first load, the titleView maybe nil
[[self subviews] enumerateObjectsUsingBlock:^(UIView *obj, NSUInteger idx, BOOL *stop) {
if ([obj isKindOfClass:NSClassFromString(@"UINavigationItemView")]) {
obj.alpha = alpha;
*stop = YES;
}
}];
}`

To:
`- (void)lt_setElementsAlpha:(CGFloat)alpha
{
self.tintColor = [self.tintColor colorWithAlphaComponent:alpha];

UIView *titleView = [self valueForKey:@"_titleView"];
titleView.alpha = alpha;

// when viewController first load, the titleView maybe nil
[[self subviews] enumerateObjectsUsingBlock:^(UIView *obj, NSUInteger idx, BOOL *stop) {
if ([obj isKindOfClass:NSClassFromString(@"UINavigationItemView")]) {
obj.alpha = alpha;
*stop = YES;
}
}];
}`