Go-Ethereum rpc.Client#Unsubscribe blocks indefinitely

when called simultaneously with geth node shutdown:

Repro steps:

To help emulate the environment needed to reproduce the bug, repro.sh bootstraps, runs and kills geth and main.go.

1. ./repro.sh
2. ps aftwerwards and ensure the go executable (ending with blockingbuild) is still running. Note that it will not respond to a regular kill command.
3. Install dlv (go get -u github.com/derekparker/delve/cmd/dlv)
4. Use dlv attach <pid> <binarypath> to attach to process
5. List goroutines with goroutines
6. Switch to goroutine stuck on go-ethereum send, i.e. goroutine 1
7. Look at call stack with stack

This issue is indeed spurious, so step 2 won't always reproduce, but in practice it reproduced 75% of the time.