[ObjCRuntime] Work around a difference in .NET where Type.IsInterface…

… doesn't work the same way for byref types.

Ref: dotnet/runtime#39068

Fixes this test:

    MonoTouchFixtures.ObjCRuntime.RegistrarTest
        [FAIL] RefOutTest_INSCoding : System.InvalidCastException : Unable to cast object of type 'Foundation.NSString' to type 'Foundation.INSCoding&'.
            at ObjCRuntime.Runtime.GetINativeObject(IntPtr ptr, Boolean owns, Type target_type, Type implementation)
            at ObjCRuntime.Runtime.GetINativeObject(IntPtr ptr, Boolean owns, Type target_type)
            at ObjCRuntime.Runtime.GetINativeObject_Dynamic(IntPtr ptr, Boolean owns, IntPtr type_ptr)
            at ObjCRuntime.Runtime.get_inative_object_dynamic(IntPtr obj, Boolean owns, IntPtr type, Int32& exception_gchandle)
            --- End of stack trace from previous location ---
            at MonoTouchFixtures.ObjCRuntime.RegistrarTest.RefOutTest_INSCoding() in /Users/rolf/work/maccore/squashed-onedotnet/xamarin-macios/tests/monotouch-test/ObjCRuntime/RegistrarTest.cs:line 2908
            at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

src/ObjCRuntime/Runtime.cs

@@ -1358,7 +1358,14 @@ static Type LookupINativeObjectImplementation (IntPtr ptr, Type target_type, Typ
 				}
 			}
 
-			if (implementation.IsInterface)
+			var interface_check_type = implementation;
+#if NET
+			// https://github.com/dotnet/runtime/issues/39068
+			if (interface_check_type.IsByRef)
+				interface_check_type = interface_check_type.GetElementType ();
+#endif
+
+			if (interface_check_type.IsInterface) 
 				implementation = FindProtocolWrapperType (implementation);
 
 			return implementation;
@@ -1382,8 +1389,14 @@ static INativeObject GetINativeObject (IntPtr ptr, bool owns, Type target_type,
 			}
 
 			if (o != null) {
+				var interface_check_type = target_type;
+#if NET
+				// https://github.com/dotnet/runtime/issues/39068
+				if (interface_check_type.IsByRef)
+					interface_check_type = interface_check_type.GetElementType ();
+#endif
 				// found an existing object, but with an incompatible type.
-				if (!target_type.IsInterface) {
+				if (!interface_check_type.IsInterface) {
 					// if the target type is another class, there's nothing we can do.
 					throw new InvalidCastException (string.Format ("Unable to cast object of type '{0}' to type '{1}'.", o.GetType ().FullName, target_type.FullName));
 				}
@@ -1456,6 +1469,11 @@ public static T GetINativeObject<T> (IntPtr ptr, bool forced_type, bool owns) wh
 
 		private static Type FindProtocolWrapperType (Type type)
 		{
+#if NET
+			// https://github.com/dotnet/runtime/issues/39068
+			if (type.IsByRef)
+				type = type.GetElementType ();
+#endif
 			if (type == null || !type.IsInterface)
 				return null;