去除UINavigationBar的下边框
[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc] init] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];//设置背景
[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]]; //覆盖边框阴影
[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc] init] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];//设置背景
[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]]; //覆盖边框阴影
