Fix worker_test failure under Ruby 1.8.7

Resque test failed under Ruby 1.8.7 env. https://travis-ci.org/resque/resque/jobs/6367594

The reason why worker_test failed under 1.8.7 is `Hash#to_s` in 1.8.7
is different from other versions(1.9/2.0).

`{:a => 1}.to_s` under 1.8.7 is `a1`, but under 1.9/2.0 is `{:a=>1}`.

And also, the original way to new a resque worker seems not right,
we should pass either an array of queues' names or just one name,
but not a Hash object.

cc @steveklabnik @sshingler