Add attribute multipart_form to runscope_step

[alxrem]

{
  "meta": {
    "status": "success"
  },
  "data": {
    "id": "25245784-7c21-4497-81d1-c3d5e60bf6ae",
    "step_type": "request",
    "skipped": false,
    "note": "",
    "method": "POST",
    "body": "",
    "form": {},
    "multipart_form": [
      {
        "name": "a",
        "value": "1",
        "is_hash": false,
        "hash": "",
        "filename": "",
        "content_type": "application/octet-stream"
      },
      {
        "name": "log",
        "value": "",
        "is_hash": true,
        "hash": "ed86e19c-e9b2-449f-a51a-f940e2a5dfe1",
        "filename": "log.rtf",
        "content_type": "application/octet-stream"
      },
      {
        "name": "a",
        "value": "2",
        "is_hash": false,
        "hash": "",
        "filename": "",
        "content_type": ""
      }
    ],
    "headers": {
      "Content-Type": [
        "multipart/form-data"
      ]
    },
    "url": "https://yourapihere.com/example/path",
    "auth": {},
    "assertions": [],
    "variables": [],
    "scripts": [],
    "before_scripts": []
  },
  "error": null
}