Add a test for #1922

Author: bblanchon

extras/tests/JsonVariant/converters.cpp

@@ -152,3 +152,16 @@ TEST_CASE("ConverterNeedsWriteableRef") {
   CHECK(ConverterNeedsWriteableRef<JsonArray>::value == true);
   CHECK(ConverterNeedsWriteableRef<JsonArrayConst>::value == false);
 }
+
+namespace ArduinoJson {
+void convertToJson(char c, JsonVariant var) {
+  char buf[] = {c, 0};
+  var.set(buf);
+}
+}  // namespace ArduinoJson
+
+TEST_CASE("Convert char to string") {  // issue #1922
+  StaticJsonDocument<64> doc;
+  doc.set('a');
+  REQUIRE(doc.as<std::string>() == "a");
+}