Fix variable names in rescue blocks

[kitop]

When running the gem in production, we came across the following error:

09:24:55.681 .../bundle/ruby/2.5.0/gems/ddtrace-0.13.0/lib/ddtrace/workers.rb:46:in `rescue in callback_traces': undefined local variable or method `items' for #<Datadog::Workers::AsyncTransport:0x000055cecd711640> (NameError)
09:24:55.681 .../bundle/ruby/2.5.0/gems/ddtrace-0.13.0/lib/ddtrace/workers.rb:38:in `callback_traces'
09:24:55.681 .../bundle/ruby/2.5.0/gems/ddtrace-0.13.0/lib/ddtrace/workers.rb:111:in `block in perform

And noticed that this commit changed the variable name in the method body but not in the rescue block.

This PR updates the variable names to the proper ones in rescue blocks.

[delner]

Hey @kitop, thanks for the PR! This looks like a pretty clear cut bug, and the fix looks good. It'd be a good idea to add an RSpec test that replicates this issue so we can confirm the efficacy of the change, and prevent any regressions. Once we have one of those, I think we can merge this one.

[kitop]

@delner thanks for the feedback! Will add tests for it.

[kitop]

@delner Added some rspec unit tests. There were already some tests for workers in test/workers_test.rb but decided to use rspec as the majority of the suite is there and it has an expectation for not raising errors (minitest doesn't :/).

Let me know if this is enough, or you'd prefer it any other way.

[delner]

Yup, RSpec is what we prefer anyways, so that's all good! I'll take a look.

[delner]

Looks good! Thanks for your contribution! This will roll with 0.13.1.