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

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

--ac
Content-Type: text/plain

body

---- EXPECTED STRUCTURE ----
{
  "headers": {
    "values": {
      "content_type": {
        "ContentType": {
          "c_type": "multipart",
          "c_subtype": "mixed",
          "attributes": {
            "boundary": "a"
          }
        }
      }
    },
    "offsets": [
      {
        "name": {
          "Rfc": "content_type"
        },
        "start": 13,
        "end": 44
      }
    ]
  },
  "attachments": [
    {
      "Attachment": 0
    }
  ],
  "parts": [
    {
      "Text": {
        "contents": "Content-Type: multipart/mixed; boundary=\"a\"\n\n--ac\nContent-Type: text/plain\n\nbody\n",
        "is_html": false
      }
    }
  ],
  "structure": {
    "Part": 0
  },
  "offset_header": 0,
  "offset_body": 45,
  "offset_end": 131
}