Silences this warning:
```
An expectation of :service_name= was set on nil. Called from /Users/abarciauskas/Projects/mdsol_services/zipkin-tracer/spec/lib/zipkin-tracer/process_tracer_spec.rb:9:in `block (2 levels) in <top (required)>'. Use allow_message_expectations_on_nil to disable warnings.
```