Unexpected 'collectionFormat' writing Swagger 2 #1053

var doc = new OpenApiDocument
{
    Paths = new OpenApiPaths
    {
        ["/foo"] = new()
        {
            Operations = new ConcurrentDictionary<OperationType, OpenApiOperation>
            {
                [OperationType.Get] = new()
                {
                    Parameters = new List<OpenApiParameter>
                    {
                        new()
                        {
                            In = ParameterLocation.Query,
                            Schema = new OpenApiSchema{Type = "string"}
                        }
                    }
                }
            }
        }
    }
};
var sb     = new StringBuilder();
var writer = new OpenApiYamlWriter(new StringWriter(sb, new CultureInfo("en_us")));

doc.SerializeAsV2(writer);

File.WriteAllText(writeFile, sb.ToString());

swagger: '2.0'
info: { }
paths:
  /foo:
    get:
      parameters:
        - in: query
          type: string
      responses: { }

swagger: '2.0'
info: { }
paths:
  /foo:
    get:
      parameters:
        - in: query
          type: string
          collectionFormat: multi
      responses: { }