fix missing client arg

[Ethan-Arrowood]

closes #554

doc update included in #550

[ronag]

Would be nice with a test. Is this the only occurance?

[Ethan-Arrowood]

Yeah only occurrence (did a search for emit('disconnect across the whole repo), most of the tests use neither arguments in the event listener, but there is one test that checks them: https://github.com/nodejs/undici/blob/master/test/pool.js#L39

[ronag]

In the upgrade test can you add a check for disconnect + correct args. I think you can just register a handler in one of the existing tests.

[mcollina]

Can you please add a unit test?

[Ethan-Arrowood]

Also found another test for this but its not upgrade related so this new case is a good addition

https://github.com/nodejs/undici/blob/master/test/client.js#L980

[mcollina]

lgtm

[mcollina]

I think this is semver-major cc @ronag.

[ronag]

It is semver major. A fix for #544 which we haven't released,