let dataTask = session.dataTask(with: urlRequest) { responseData, urlResponse, err -> Void in
     let data: Data = responseData
}

func parse(data: Data) -> Bool? {
    switch String(data: responseBody, encoding: .utf8) {
    case "true"?:
        return true
    case "false"?:
        return false
    default:
        return nil
    }
}

let responseString = "false"
let responseBody = responseString.data(using: .utf8)!

parse(data: responseBody)