[registrar] BindAs uses Nullable types so allow them to be registered as NSObjects

BindAsAttribute allows to bind NSValue and NSNumber into more
accurate C# types lyke bool?, int? etc. so we must teach registrar
about this.

src/ObjCRuntime/Registrar.cs

@@ -2130,7 +2130,9 @@ namespace XamCore.Registrar {
 		{
 			bool isNativeEnum;
 
-			switch (GetTypeFullName (type)) {
+			var typeFullName = GetTypeFullName (type);
+
+			switch (typeFullName) {
 			case "System.IntPtr": return "^v";
 			case "System.SByte": return "c";
 			case "System.Byte": return "C";
@@ -2163,6 +2165,10 @@ namespace XamCore.Registrar {
 				throw CreateException (4102, member, "The registrar found an invalid type `{0}` in signature for method `{2}`. Use `{1}` instead.", "System.DateTime", IsDualBuild ? "Foundation.NSDate" : CompatNamespace + ".Foundation.NSDate", member.FullName);
 			}
 
+			// We use BindAsAttribute to wrap NSNumber/NSValue into more accurate Nullable<T> types
+			if (typeFullName != null && typeFullName.Contains ("Nullable"))
+				return "@";
+
 			if (Is (type, ObjCRuntime, "Selector"))
 				return ":";