Retry sometimes fail

[slinkydeveloper]

Describe the bug
Sometimes retry fails and lose the event with reason:

imc-dispatcher-5d56775685-j855p dispatcher 2020-10-28T13:39:57.485Z	error	inmemorychannel-dispatcher.knative.dev-eventing-pkg-reconciler-inmemorychannel-dispatcher.Reconciler	fanout/fanout_message_handler.go:284	Fanout had an error	{"commit": "02a30e0", "knative.dev/pod": "imc-dispatcher-5d56775685-j855p", "knative.dev/traceid": "de5a0a45-3a68-4049-bca3-9ecfd55b8419", "knative.dev/key": "foobar/channel", "error": "unable to complete request to http://receiver.foobar.svc.cluster.local/: Post \"http://receiver.foobar.svc.cluster.local/\": http: ContentLength=46215 with Body length 0"}
imc-dispatcher-5d56775685-j855p dispatcher knative.dev/eventing/pkg/channel/fanout.(*FanoutMessageHandler).dispatch
imc-dispatcher-5d56775685-j855p dispatcher 	knative.dev/eventing/pkg/channel/fanout/fanout_message_handler.go:284
imc-dispatcher-5d56775685-j855p dispatcher knative.dev/eventing/pkg/channel/fanout.createMessageReceiverFunction.func1.1
imc-dispatcher-5d56775685-j855p dispatcher 	knative.dev/eventing/pkg/channel/fanout/fanout_message_handler.go:193

Expected behavior
This failure should not happen and retry should properly work

Knative release version
0.17, 0.18, master

[slinkydeveloper]

/assign