push(x) after push(null) should cause an error

[martinheidegger]

Similarly to #44, the following test currently fails:

tape('push after push(null) should cause an error', function (t) {
  t.plan(5)
  const s = new Readable()
  s.on('data', function (data) {
    t.equals(data, 'a')
  })
  s.on('error', function () {
    t.pass('stream.push() after EOF')
  })
  s.on('close', function () {
    t.end()
  })
  t.equals(s.push('a'), true)
  t.equals(s.push(null), false)
  t.equals(s.push('b'), false)
})

Both pushed data statements are passed through and no error like stream.push() after EOF as this is an incompatibility to the current Node.js streams and am wondering: is that intentional? should that be documented?

[mafintosh]

Yea basically just reflection the simplicity point here, https://github.com/streamxorg/streamx#simplicity
If trivial to add, feel free to PR.