Commented Unassigned: IntegerExtension.ToStringLookup not threadsafe [9118]

Hi,
We are using this library in our service which generates several reports in parallel threads and we're getting the following exceptions:

System.ArgumentException: An item with the same key has already been added.
at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at ClosedXML.Excel.IntegerExtensions.ToStringLookup(Int32 value)

This error is caused by those lines of code (of static class ClosedXML.Excel.IntegerExtensions):
private static readonly Dictionary<int, string> intToString = new Dictionary<int, string>();

public static string ToStringLookup(this int value)
{
if (!IntegerExtensions.intToString.ContainsKey(value))
IntegerExtensions.intToString.Add(value, value.ToString((IFormatProvider) IntegerExtensions.nfi));
return IntegerExtensions.intToString[value];
}

I think replace this dictionary by ConcurrentDictionary and use AddOrUpdate method will do the trick :)

Regards,
Maciej.
Comments: I did some quick testing, I am not even sure the caching in this case is getting us much of a benefit. I ran multiple test, and it always seems faster to just to string the int on my machine, then it does to do the dictionary lookup. Not to mention, if there are many numbers, I could watch the memory grow in task manager. It may help our memory consumption to ditch this cache. Especially for large workbooks. What are your thoughts. Would you like me to proceed with this change?