IsAssignableTo is internal and never called with null

There is a IsAssignableTo in .NET 5.0 - the implementation is different. We need to clean the uses of this method if we want to expose it. Which is why I opened #151

Framework.Core/System/Dynamic/Utils/TypeUtils.cs

@@ -213,7 +213,7 @@ internal static bool IsArrayTypeAssignableTo(Type type, Type target)
                 return false;
             }
 
-            return type.GetArrayRank() == target.GetArrayRank() && type.GetElementType().IsAssignableToInternal(target.GetElementType());
+            return type.GetArrayRank() == target.GetArrayRank() && type.GetElementType().IsAssignableTo(target.GetElementType());
         }
 
         internal static bool IsArrayTypeAssignableToInterface(Type type, Type target)
@@ -233,21 +233,6 @@ internal static bool IsArrayTypeAssignableToInterface(Type type, Type target)
         }
 
         internal static bool IsAssignableTo(this Type type, Type target)
-        {
-            if (type == null)
-            {
-                throw new ArgumentNullException(nameof(type));
-            }
-
-            if (target == null)
-            {
-                throw new ArgumentNullException(nameof(target));
-            }
-
-            return type.IsAssignableToInternal(target);
-        }
-
-        internal static bool IsAssignableToInternal(this Type type, Type target)
         {
             return target.IsAssignableFrom(type)
                    || IsArrayTypeAssignableTo(type, target)