throws in onSend hook never come back

[BigMurry]

onSend hook may has a cycle execution issue when calling next(new Error('some error')). For example:

fastify.addHook('onSend', function (request, reply, payload, next) {
  next(new Error('some error'));
})
fastify.get('/', function (request, reply) {
  // it will never get here.
})

if we start a GET / request, it'll timeout
As the current execution sequence is:
onSend next(some error) -> wrapOnSendEnd -> onSendEnd -> handleError -> onSend hook
If onSend throws error again, the upper execution will start again.

Create a PR #410