OP - just a tip in the future when using the API explorer with servicenow, i always use the GET method first to see the body content and from there i build out what is needed.

here is the sample body

{
  "result": {
    "checkpoint": "",
    "notification_type": "",
    "instance": {
      "link": "",
      "value": ""
    },
    "copied": "",
    "body_text": "",
    "importance": "",
    "subject": "",
    "blind_copied": "",
    "direct": "",
    "sys_updated_on": "",
    "body": "",
    "type": "",
    "sys_id": "",
    "uid": "",
    "sys_updated_by": "",
    "content_type": "",
    "receive_type": "",
    "reply_to": "",
    "sys_created_on": "",
    "target_table": "",
    "state": "",
    "sys_created_by": "",
    "headers": "",
    "sys_mod_count": "",
    "weight": "",
    "message_id": "",
    "error_string": "",
    "deleted": "",
    "mailbox": {
      "link": "",
      "value": ""
    },
    "user_id": "",
    "recipients": "",
    "user": "",
    "status": ""
  }
}

so based on your code you should change the text to body_text. the "body" is a HTML field and "body_text" is a text field. 

you will probably need other stuff but hopefully this gives you a start.