Fix reusing ASIO http_client connecting to HTTPS server via proxy (#539)

Calling request() twice on the same client configured to connect to a
server via HTTPS didn't work under Unix because we issues CONNECT for
every request, meaning that, for the second one, we sent CONNECT via an
already established connection to the end server itself which,
unsurprisingly, didn't work at all.

Fix this by only setting up SSL tunnelling for a new connection but not
for the already used one.

Release/src/http/client/http_client_asio.cpp

@@ -905,7 +905,8 @@ class asio_context final : public request_context, public std::enable_shared_fro
             }
         };
 
-        if (proxy_type == http_proxy_type::ssl_tunnel)
+        // Note that we must not try to CONNECT using an already established connection via proxy -- this would send CONNECT to the end server which is definitely not what we want.
+        if (proxy_type == http_proxy_type::ssl_tunnel && !m_connection->is_reused())
         {
             // The ssl_tunnel_proxy keeps the context alive and then calls back once the ssl tunnel is established via
             // 'start_http_request_flow'