How to write unit test for network client?

I need to write simple http client. It could be great to have unit tests for my class. But I don't know how to write proper and testable class.

For example, I have a client like this:

class HTTPClient
{
public:
     HTTPCLient(const std::string& host, const std::string& port): session(host, port) {}

     void send()
     {
         session.sendRequest(someRequest);
         Response response = session.receiveResponse();
         // ...
     }

private:
    SomeLibrary::ClientSession session;
};

How to test send method (that I really send what I want)? I can't mock it. I can write that HTTPClient receives SomeLibrary::ClientSession object in constructor (in test I would pass mock) but is it good design? I think that the way of implementing of session etc. should by hidden in my class.

Do You have any idea?