Fix worker_test failure under Ruby 1.8.7

* 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.
Merge pull request #949 from larrylv/fix-worker-test-failure-under-1.8.7

Fix worker_test failure under Ruby 1.8.7