Operations with body and form params should be disallowed

[gheine]

The request body can either contain a json body or be form encoded. Thus mixing the two should not be possible. Example:

{
  "name": "test",
  "base_url": "http://localhost.com",
  "apidoc": {
    "version": "0.11.48"
  },
  "models": {
    "test": {
      "fields": [
        {
          "name": "id",
          "type": "string"
        }
      ]
    }
  },
  "resources": {
    "test": {
      "operations": [
        {
          "method": "POST",
          "path": "/foo",
          "parameters": [
            {
              "name": "bar",
              "type": "string",
              "location": "form"
            }
          ],
          "body" : {
            "type" : "test"
          },
          "responses": {
            "204": { "type": "unit" }
          }
        }
      ]
    }
  }
}