null values are being inserted into the GetAllowedTypes result set

When a ReflectionTypeLoadException occurs, the Types property may contain null values.  These null values are being inserted directly into the result set in GetAllowedTypes.

Explanation for why nulls exist: http://msdn.microsoft.com/en-us/library/system.reflection.reflectiontypeloadexception.types(v=vs.110).aspx

I also noticed the Types property was bypassing the AssemblyScanner.IsAllowedType check, so I placed the null check in that method and then ran the Types property through it to follow the same semantics used when no exception thrown.