Use firstWeekday and from current calendar if not set

In application I'm working on I need to work with the first day of week for a given date. I'm getting that value with `mt_startOfCurrentWeek`. But recently I've noticed the problem when in user's calendar week starts from Monday (i.e/ United Kingdom). `mt_startOfCurrentWeek` was returning Sunday as the beginning of week like for USA.

As a workaround I can set up first weekday before using MTDates:
```objective-c
[NSDate mt_setFirstDayOfWeek:[[NSCalendar currentCalendar] firstWeekday]]
```

But I don't think it's a correct way. I think it would be better if MTDates will just work out of the box with current user's preferences...

I don't know how better to prove this with unit tests... Here is the list of steps for manual testing:

1. switch region format to "United Kingdom" on a device or a simulator (Settings => General => International => Region format)
1. log `[[NSDate date] mt_startOfCurrentWeek]` somewhere in the app with MTDates included.

Expected: the date should be Monday.
Result: always Sunday.