fix rickytan#168 ，优先使用 navigationItem.title，与系统行为一致

VC 的默认行为：当在 viewDidLoad 中设置 self.title 时，同时会更新  self.navigationItem.title，但如果在 initWithCoder （xib）中设置 title，这里 navigationItem 还没有初始化，所以无效。这里保持兼容，取不到 navigationItem.title 时，取 title

Author: rickytan

RTRootNavigationController.podspec

@@ -8,7 +8,7 @@
 
 Pod::Spec.new do |s|
   s.name             = 'RTRootNavigationController'
-  s.version          = '0.6.4'
+  s.version          = '0.6.5'
   s.summary          = 'Transparently make every view controller has its own navigation bar'
   s.description      = <<-DESC
 More and more apps use custom navigation bar for each different view controller, instead of one common, gloabal navigation bar.

RTRootNavigationController/Classes/RTRootNavigationController.m

@@ -669,7 +669,7 @@ - (void)pushViewController:(UIViewController *)viewController
                                                            viewController.rt_navigationBarClass,
                                                            self.useSystemBackBarButtonItem,
                                                            currentLast.navigationItem.backBarButtonItem,
-                                                           currentLast.title)
+                                                           currentLast.navigationItem.title ?: currentLast.title)
                          animated:animated];
     }
     else {