I saw this added in Rx 2.2.2: http://rx.codeplex.com/SourceControl/latest#Rx.NET/Source/System.Reactive.Linq/Reactive/Linq/Observable/GroupByUntil.cs

I thought it was about limiting the groups, but apparently the [ConcurrentDictionary](http://msdn.microsoft.com/en-us/library/dd287173.aspx) uses the capacity to specify the concurrency level, not the group number.

I guess this PR can be ignored.