add the class converter for the option. 
```from_string_converter=class_converter``` 

having it as a string "future proofs" the code.  If ConnectionContext were imported as in peterbe's referenced code, you'd be hardcoding a RabbitMQ dependency into the file.  In a year when we drop RabbitMQ in favor of SunshineUnicornMQ, we'll don't want to have to load the RabbitMQ libraries even though we're not using them.