Result: allow default constructor

[vmaffione]

Allow default constructor for Result, to support use cases such as the following:

       [...]
       httplib::Client client(host);
       httplib::Result result; // needed here

        if (method == "GET") {
                result = client.Get(path);

        } else if (method == "POST") {
                result = client.Post(path, headers, body,
                                "application/json");
         }

         if (!result) {
             std::cout << "Error";
             return -1;
         }

         // use result
         [...]

[yhirose]

@vmaffione thanks for your contribution!

[vmaffione]

don't say that or you'll get more :)

Thanks for accepting the patch.