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

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

--ac
Content-Type: text/plain

body

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