Edited Issue: extract_json() memory leak Reported [78]

Hi.

I've used v1.4 in my VC++(MFC) project. ( i solved dll unloaded problem - https://casablanca.codeplex.com/workitem/17 )
And I get a memory leak report.

I tested SDI Project with the following code in VS2013, VS2012

// URI
web::http::uri_builder request_uri;
request_uri.set_scheme(L"http");
request_uri.set_host(m_hostAddress.c_str());
request_uri.set_path(L"/user/login");
request_uri.append_query(L"apiKey", m_strAppKey.c_str());
request_uri.append_query(L"email", userID.c_str());
request_uri.append_query(L"passwd", userPassword.c_str());

// Request Message
web::http::http_request request(web::http::methods::POST);
request.set_request_uri(request_uri.to_string());

http_client client(request.request_uri());

client.request(request.method()).then([&](pplx::task<http_response> responseTask)
{
try
{
http_response response = responseTask.get();

if(response.status_code() == status_codes::OK)
{

pplx::task<web::json::value> previousTask = response.extract_json();
}
}
catch( const http_exception &e )
{

}
}).wait();

The same probliem appears in extract_string().