Testing NSURLSession "resume cannot be sent to abstract instance of class NSURLSessionDataTask"

I want to test NSURLSession but I get the error "resume cannot be sent to abstract instance of class NSURLSessionDataTask".

Example code is in https://github.com/stevencurtis/abstract-instanceofclassNSURLSessionDataTask

My HTTP Manager works fine:

class HTTPManager {

    static let shared: HTTPManager = HTTPManager()

    private let session: URLSessionProtocol

    init(session: URLSessionProtocol = URLSession.shared) {
        self.session = session

    }

    public func get(urlString: String, completionBlock: ((Data?) -> Void)?) {

        let url = URL(string: urlString)
        if let usableUrl = url {
            let request = URLRequest(url: usableUrl)
            let task = session.dataTask(with: request, completionHandler: { (data, response, error) in
                (completionBlock?(data))!
            })
            task.resume()
        }
    }

}

However I try to mock it

class MockURLSession: URLSessionProtocol {
    func dataTask(with request: URLRequest, completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask {
        return URLSessionDataTask()
    }
    var searchedURL = URL(string: "asd")
}

And then create the test:

func testGetRequest() {
    let url = URL(string: "http://gojek-contacts-app.herokuapp.com/contacts.json")
    let session = MockURLSession()
    let sub = HTTPManager(session: session)


    sub.get(urlString: "http://gojek-contacts-app.herokuapp.com/contacts.json", completionBlock: { [weak self] (data: Data?) -> Void in
        print ("vc")
        }
    )
}

The tests errors with: resume cannot be sent to abstract instance of class NSURLSessionDataTask" and I can't think of a way around this error!