Fix cancellation of redirected requests

[clue]

While working on an implementation for clue/reactphp-mq#6, I noticed that some requests do no seem to be cancelled properly, i.e. they keep running despite their promises being cancelled. A similar issues came up in clue/reactphp-mq#7 as it relies on this project for its example. It turns out that this affects cancellation of requests that are issued after the initial request is successfully completed (i.e. redirected requests). Cancellation of operations that have been started instantly worked as expected.

This will eventually also be addressed upstream via reactphp/promise#99, but it's currently not decided when this version will be tagged.