Fix GCC 7 warning

src/ArduinoJson/Serialization/JsonSerializerImpl.hpp

@@ -90,7 +90,8 @@ inline void ArduinoJson::Internals::JsonSerializer<Writer>::serialize(
       return;
 
     case JSON_NEGATIVE_INTEGER:
-      writer.writeRaw('-');
+      writer.writeRaw('-');  // Falls through.
+
     case JSON_POSITIVE_INTEGER:
       writer.writeInteger(variant._content.asInteger);
       return;