Fix when multipart has body of multipart stream

[uikolas]

Problem happens, when I try to use multipart.

Example:

    "PostTicketRequest": {
      "httpMethod": "POST",
      "uri": "/api/...",
      "summary": "Posts ticket request",
      "responseModel": "Response",
      "parameters": {
        "description": {
          "location": "multipart",
          "type": "string",
          "required": true
        },
        ....

The correct header is not added, since request is immutable object. So we need to assign a variable. Which was missed.