Support returning any from callbacks

Add test

Fix test

callback.go

@@ -353,13 +353,27 @@ func callbackRetNil(ctx *C.sqlite3_context, v reflect.Value) error {
 	return nil
 }
 
+func callbackRetAny(ctx *C.sqlite3_context, v reflect.Value) error {
+        cb, err := callbackRet(v.Elem().Type())
+        if err != nil {
+                return err
+        }
+
+        return cb(ctx, v.Elem())
+}
+
 func callbackRet(typ reflect.Type) (callbackRetConverter, error) {
 	switch typ.Kind() {
 	case reflect.Interface:
 		errorInterface := reflect.TypeOf((*error)(nil)).Elem()
 		if typ.Implements(errorInterface) {
 			return callbackRetNil, nil
 		}
+
+		if typ.NumMethod() == 0 {
+			return callbackRetAny, nil
+		}
+
 		fallthrough
 	case reflect.Slice:
 		if typ.Elem().Kind() != reflect.Uint8 {

callback_test.go

@@ -102,3 +102,15 @@ func TestCallbackConverters(t *testing.T) {
 		}
 	}
 }
+
+func TestCallbackReturnAny(t *testing.T) {
+	udf := func() interface{} {
+		return 1
+	}
+
+	typ := reflect.TypeOf(udf)
+	_, err := callbackRet(typ.Out(0))
+	if err != nil {
+		t.Errorf("Expected valid callback for any return type, got: %s", err)
+	}
+}