Add support for namespaced workers (e.g. Foo::Bar)

Originally I had the code written like

```ruby
w = if k.respond_to?(:constantize)
  k.constantize
else
  k.split('::').inject(Kernel){|s, c| s.const_get(c)}
end
```

so it could lean on ActiveSupport/Rails if available, but wasn't sure how to properly cover that in the test code so I just took it out.