fix(listen): makes sure unlisten returns a reference to conn result

[stalniy]

Eventually, this allows to get the new pid of pg connection after listen reconnects.

Follow up PR for #155
Issue in tests: how can I test unequality in tests?

t('listen result reports correct connection state after reconnection', async() => {
  const listener = postgres(options)
      , xs = []

  const result = await listener.listen('test', x => xs.push(x))
  const initialPid = result.state.pid
  await sql.notify('test', 'a')
  await sql`select pg_terminate_backend(${ initialPid }::int)`
  await delay(50)
  listener.end()

  return [result.state.pid, initialPid]
})

[porsager]

Cool @stalniy !

To test inequality you can simply do [this !== that, true]

[stalniy]

right! So simple :)

[stalniy]

@porsager any plans to merge and release this?

[porsager]

Ah, sorry. Didn't see your latest changes. Looks good.