The `content-type` for the MQTT protocol might have been set multiple times

[yanmxa]

When setting the event transfer with structured mode by MQTT protocol. The content-type might be set more than one times, see the MQTT payload below.

• Sender with structured-mode

  ...
  ctx = cloudevents.WithEncodingStructured(ctx)
  if result := c.Send(
  cecontext.WithTopic(ctx, "test-topic"),
  e,
  ); cloudevents.IsUndelivered(result) {
  log.Printf("failed to send: %v", result)
  } else {
  log.Printf("sent: %d, accepted: %t", i, cloudevents.IsACK(result))
  }

• The MQTT Payload

  {
  "PacketID": 0,
  "QoS": 0,
  "Retain": false,
  "Topic": "test-topic",
  "Properties": {
  "CorrelationData": null,
  "ContentType": "",
  "ResponseTopic": "",
  "PayloadFormat": null,
  "MessageExpiry": null,
  "SubscriptionIdentifier": null,
  "TopicAlias": null,
  "User": [
    {
      "Key": "Content-Type",
      "Value": "application/cloudevents+json"
    },
    {
      "Key": "Content-Type",
      "Value": "application/cloudevents+json"
    },
    {
      "Key": "Content-Type",
      "Value": "application/cloudevents+json"
    },
    {
      "Key": "Content-Type",
      "Value": "application/cloudevents+json"
    }
  ]
  },
  "Payload": "eyJzcGVjdmVyc2lvbiI6IjEuMCIsImlkIjoiNTVkM2NhYTktOTA2ZC00MDE0LWI3YTYtZDUyOTA3NjAxZTE2Iiwic291cmNlIjoiaHR0cHM6Ly9naXRodWIuY29tL2Nsb3VkZXZlbnRzL3Nkay1nby9zYW1wbGVzL21xdHQvc2VuZGVyIiwidHlwZSI6ImNvbS5jbG91ZGV2ZW50cy5zYW1wbGUuc2VudCIsImRhdGFjb250ZW50dHlwZSI6ImFwcGxpY2F0aW9uL2pzb24iLCJ0aW1lIjoiMjAyNC0wNS0yM1QwMjowNTozMS4zMzU5NzNaIiwiZGF0YSI6eyJpZCI6MywibWVzc2FnZSI6IkhlbGxvLCBXb3JsZCEifSwiZXZlbnRrZXkiOiJldmVudHZhbHVlIn0="
  }

So instead of setting the content-type in the userProperties list, we can leverage the ContentType in the MQTT properties directly.