call unregisterConsumer in handleCancel

[Uzlopak]

This PR adds a call to unregisterConsumer in handleCancel. This ensures, that the consumer gets removed from this.consumers.

Fixes #585

[kibertoad]

Can you also add a test for this?

[Uzlopak]

@kibertoad
I have to see if I can write a unit test. I think when I implemented this I thought that @squaremo would check if the code makes change.

Do you have the permissions to merge?
Alot of stuff we could be improved.

Have a look at #658 . It think this could actually be solve some memory issues as it should potentially not create intermediate arrays.

[kibertoad]

@Uzlopak I do!