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

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

--a----
Content-Type: text/plain

body
--a------
Content-Type: text/html

body2
--a----
---- EXPECTED STRUCTURE ----{
  "html_body": [
    2
  ],
  "text_body": [
    2
  ],
  "attachments": [],
  "parts": [
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "multipart",
            "c_subtype": "mixed",
            "attributes": {
              "boundary": "a--"
            }
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 13,
            "end": 46
          }
        ]
      ],
      "is_encoding_problem": false,
      "body": {
        "Multipart": [
          1
        ]
      },
      "offset_header": 0,
      "offset_body": 47,
      "offset_end": 189
    },
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "multipart",
            "c_subtype": "mixed",
            "attributes": {
              "boundary": "a----"
            }
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 66,
            "end": 101
          }
        ]
      ],
      "is_encoding_problem": false,
      "body": {
        "Multipart": [
          2
        ]
      },
      "offset_header": 53,
      "offset_body": 102,
      "offset_end": 151
    },
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "text",
            "c_subtype": "plain",
            "attributes": null
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 123,
            "end": 135
          }
        ]
      ],
      "is_encoding_problem": false,
      "body": {
        "Text": "body"
      },
      "offset_header": 110,
      "offset_body": 136,
      "offset_end": 141
    }
  ]
}