Fix type of publishable payload: it's iodata()|nil, not binary()|nil

[nabijaczleweli]

These get fed into

%Package.Publish{
  topic: topic,
  qos: qos,
  payload: payload,
  retain: Keyword.get(opts, :retain, false)
}

then

encoded_publish = Package.encode(publish)

which matches

defmodule Tortoise.Package.Publish do
  defimpl Tortoise.Encodable do
    def encode(%Package.Publish{}) do
      [
        ...,
        Package.variable_length_encode([
          ...,
          payload
        ])
      ]

which already forms an iodata()

Testing confirms this result

Forwarded-from: gausby#148
Sponsored-by: https://beaverlabs.net