cancelButtonText 和 confirmButtonText 设置无效

[YQqiang]

分析：
设置 取消 确定按钮文案的方法有两处：

1. 在 setLanguage: 方法中

- (void)setLanguage:(NSString *)language {
    _language = language;
    
    NSString *cancelButtonText = [NSBundle pg_localizedStringForKey:@"cancelButtonText" language:self.language];
    [self.cancelButton setTitle:cancelButtonText forState:UIControlStateNormal];
    
    NSString *confirmButtonText = [NSBundle pg_localizedStringForKey:@"confirmButtonText" language:self.language];
    [self.confirmButton setTitle:confirmButtonText forState:UIControlStateNormal];
}

2. 在 setupButton 方法中

- (void)setupButton {
    [self.cancelButton setTitle:self.cancelButtonText forState:UIControlStateNormal];
    [self.confirmButton setTitle:self.confirmButtonText forState:UIControlStateNormal];
}

setupButton的调用时机为： PGDatePickManagerHeaderView 的 layoutSubviews时；
setLanguage:的调用时机为：PGDatePickManager 的 viewDidLayoutSubviews时；

在 iOS 9.3.2系统下的表现行为是：先调用PGDatePickManagerHeaderView 的 layoutSubviews，然后调用 PGDatePickManager 的 viewDidLayoutSubviews

表现的结果是：cancelButtonText 和 confirmButtonText 无效

修复：
在 setLanguage: 中检查cancelButtonText，confirmButtonText 是否有值

- (void)setLanguage:(NSString *)language {
    _language = language;
    if (!_cancelButtonText || _cancelButtonText.length <= 0) {
        NSString *cancelButtonText = [NSBundle pg_localizedStringForKey:@"cancelButtonText" language:self.language];
        [self.cancelButton setTitle:cancelButtonText forState:UIControlStateNormal];
    }
    
    if (!_confirmButtonText || _confirmButtonText.length <= 0) {
        NSString *confirmButtonText = [NSBundle pg_localizedStringForKey:@"confirmButtonText" language:self.language];
        [self.confirmButton setTitle:confirmButtonText forState:UIControlStateNormal];
    }
}