Fix volatile bool serialized as 1 or 0

Ported from 5d1d2721d1

extras/tests/JsonVariant/types.cpp

@@ -140,6 +140,13 @@ TEST_CASE("volatile") {
   JsonDocument doc;
   JsonVariant variant = doc.to<JsonVariant>();
 
+  SECTION("volatile bool") {  // issue #2029
+    volatile bool f = true;
+    variant.set(f);
+    CHECK(variant.is<bool>() == true);
+    CHECK(variant.as<bool>() == true);
+  }
+
   SECTION("volatile int") {
     volatile int f = 42;
     variant.set(f);