title: NSCalendar, why does setting the firstWeekday doesn't effect calculation outcome?
body: <p>i need to calculate the weekday for a given date, however, depending on the calendar a week can star on Monday and somwhere on Sunday</p>
  
  <p>so i wanted to set it, to start on Monday, using</p>
  
  <pre><code>[[NSCalendar currentCalendar] setFirstWeekday:2];
  </code></pre>
  
  <p>however, the calculation outcome is the same</p>
  
  <pre><code>{
      [[NSCalendar currentCalendar] setFirstWeekday:1];
      NSDateComponents *weekdayComponents = [[NSCalendar currentCalendar] components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];
      NSInteger weekday = [weekdayComponents weekday] - 1;
      NSLog(@"%d", weekday);
  }
  {
      [[NSCalendar currentCalendar] setFirstWeekday:2];
      NSDateComponents *weekdayComponents = [[NSCalendar currentCalendar] components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];
      NSInteger weekday = [weekdayComponents weekday] - 1;
      NSLog(@"%d", weekday);
  }
  </code></pre>
  
  <p>returns same numbers, but why?</p>