Content-Type: multipart/mixed; boundary="ab"

--ab
Content-Type: multipart/mixed; boundary="a"

--ab
Content-Type: text/plain

--a


---- EXPECTED STRUCTURE ----
{
  "header": {
    "content_type": {
      "c_type": "multipart",
      "c_subtype": "mixed",
      "attributes": {
        "boundary": "ab"
      }
    }
  },
  "html_body": [
    {
      "contents": "<html><body>Content-Type: text/plain<br/></body></html>"
    }
  ],
  "text_body": [
    {
      "contents": "Content-Type: text/plain\n"
    }
  ]
}