changed queue_name to code_name

Fixing a small typo. Doesn't change underlying functionality, and tests continue to pass with the modification.

```sh
$ ruby -itest test/iron_worker_test.rb 
Run options: 

# Running tests:

...

Finished tests in 0.007968s, 376.5060 tests/s, 502.0080 assertions/s.

3 tests, 4 assertions, 0 failures, 0 errors, 0 skips
```