Add default key for limits_concurrency

[ankane]

Hi, this PR adds a default key to limits_concurrency so users don't need to come up with a key name if they only want to limit by job name (regardless of arguments), which I expect is a common case.

class SomeJob < ApplicationJob
  limits_concurrency to: 3
end

Currently, users must use an empty string or another key.

class SomeJob < ApplicationJob
  limits_concurrency key: "", to: 3
end

[ankane]

The test failure seems to be from a flaky test (also in this CI run from February).