Fix type casting issue in requirements

[ibboard]

Despite all of the unit tests passing, we currently get an error when trying to add units to armies in both UIs. The issue comes from the use of a custom extension of the ICollection class. We need to create a test to recreate it and then fix it.

(System.Reflection.TargetInvocationException) Exception has been thrown by the target of an invocation.
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidCastException: Cannot cast from source type to destination type.
  at IBBoard.WarFoundry.API.Objects.Army.GetAddingUnitRequirements (IBBoard.WarFoundry.API.Objects.UnitType addingType) [0x00000] in <filename unknown>:0 
  at IBBoard.WarFoundry.API.Objects.Requirement.RequirementHandler.AllowsAdding (IBBoard.WarFoundry.API.Objects.UnitType unitType, IBBoard.WarFoundry.API.Objects.Army army, ICollection`1& failureMessages) [0x00000] in <filename unknown>:0 
  at IBBoard.WarFoundry.GUI.GTK.FrmNewUnit.SetSelectUnitEnabledVal () [0x00019] in /home/ibboard/Projects/dev/IBBoard.WarFoundry.GUI.GTK/FrmNewUnit.cs:109 
  at IBBoard.WarFoundry.GUI.GTK.FrmNewUnit.OnSelectionChanged (System.Object o, System.EventArgs e) [0x00000] in /home/ibboard/Projects/dev/IBBoard.WarFoundry.GUI.GTK/FrmNewUnit.cs:78 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000d0] in /home/abuild/rpmbuild/BUILD/mono-2.10.6/mcs/class/corlib/System.Reflection/MonoMethod.cs:226 

[ibboard]

Can't recreate now (tried Windows, VS.Net debug build, MonoDevelop, Linux build in Windows, nightly in Windows). SimpleSet changed its class definition recently, so suspecting it was an old library that didn't get rebuilt.