LogStash::Runner pipeline settings when :pipeline_workers is not defined by the user should not pass the value to the pipeline

failure from master (hash 9cc165a428a356c974d7ae56f4789cd77a6b3f74), running `rake test:core`:

```
Failures:

1) LogStash::Runner pipeline settings when :pipeline_workers is not defined by the user should not pass the value to the pipeline
Failure/Error: expect(settings.set?("pipeline.workers")).to be(false)

expected false
got true
# ./logstash-core/spec/logstash/runner_spec.rb:261:in `(root)'
# ./logstash-core/lib/logstash/runner.rb:357:in `create_agent'
# ./logstash-core/lib/logstash/runner.rb:272:in `execute'
# ./vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'
# ./logstash-core/lib/logstash/runner.rb:185:in `run'
# ./vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'
# ./logstash-core/spec/logstash/runner_spec.rb:264:in `(root)'
# ./vendor/bundle/jruby/1.9/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `(root)'
# ./rakelib/test.rake:49:in `(root)'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/task.rb:250:in `execute'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/task.rb:250:in `execute'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/task.rb:194:in `invoke_with_call_chain'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/task.rb:187:in `invoke_with_call_chain'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/task.rb:180:in `invoke'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:152:in `invoke_task'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:108:in `top_level'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:108:in `top_level'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:117:in `run_with_threads'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:102:in `top_level'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:80:in `run'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:178:in `standard_exception_handling'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/lib/rake/application.rb:77:in `run'
# ./vendor/bundle/jruby/1.9/gems/rake-12.0.0/exe/rake:27:in `(root)'

Finished in 3 minutes 42.6 seconds (files took 5.97 seconds to load)
1711 examples, 1 failure

Failed examples:

rspec ./logstash-core/spec/logstash/runner_spec.rb:259 # LogStash::Runner pipeline settings when :pipeline_workers is not defined by the user should not pass the value to the pipeline

Randomized with seed 18296
```

The failure is not triggered consistently.