Auto merge of #19350 - gterzian:ignore_aborted_responses_in_caching, r=jdm

Ignore aborted responses in caching

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@jdm @KiChjang @Manishearth Follow up on https://github.com/servo/servo/pull/18676 and https://github.com/servo/servo/pull/19274 to ignore aborted responses in caching.

I also found out the cache shouldn't return any response whose body is still in `ResponseBody::Receiving` mode, because that fails the assertion at https://github.com/servo/servo/blob/master/components/net/fetch/methods.rs#L438(we might want to add a channel as pat of the cached response later on to deal with this case). I only found out now because I needed the response from the server to trickle in so that it could be cached and aborted.

I copied the `http-cache.py` server from the wpt folder, and added a 'trickle' option, which is necessary to actually have a failing test with a cached but aborted request, it's now passing.

I also remove one unused import that slippled through previously.

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