Duplicate ID in TimeZoneInfo

[Lunat1q]

With current implementation I face this issue:

System.TypeInitializationException: The type initializer for 'TimeZoneConverter.TZConvert' threw an exception. 
---> System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary.Insert(TKey key, TValue value, Boolean add)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable source, Func keySelector, Func elementSelector, IEqualityComparer comparer)
   at TimeZoneConverter.TZConvert.GetSystemTimeZones()
   at TimeZoneConverter.TZConvert..cctor()
   at TimeZoneConverter.TZConvert.GetTimeZoneInfo(String windowsOrIanaTimeZoneId)

According to https://referencesource.microsoft.com/#mscorlib/system/timezoneinfo.cs,2881 there could be 2 Time zone Id with same name but different case.
However, I clearly understand that registry key may not have subkeys with same name but different case. Yet this issue is reproducing. To mitigate this and prevent same issue in the future, it worth adding such distinct check.

[Lunat1q]

@mattjohnsonpint, do you have any comments regarding that?

[mattjohnsonpint]

Thanks for the PR, and sorry for the delay.

I changed the implementation slightly, to avoid a custom comparer. Also, I added a call to TimeZoneInfo.ClearCachedData before getting the system time zones, which should hopefully avoid the issue entirely.