Clear error codes idiomatically

example/http/client/async-ssl/http_client_async_ssl.cpp

@@ -191,7 +191,7 @@ public:
         {
             // Rationale:
             // http://stackoverflow.com/questions/25587403/boost-asio-ssl-async-shutdown-always-finishes-with-an-error
-            ec.assign(0, ec.category());
+            ec = {};
         }
         if(ec)
             return fail(ec, "shutdown");

example/http/client/coro-ssl/http_client_coro_ssl.cpp

@@ -113,7 +113,7 @@ do_session(
     {
         // Rationale:
         // http://stackoverflow.com/questions/25587403/boost-asio-ssl-async-shutdown-always-finishes-with-an-error
-        ec.assign(0, ec.category());
+        ec = {};
     }
     if(ec)
         return fail(ec, "shutdown");

example/http/client/sync-ssl/http_client_sync_ssl.cpp

@@ -111,7 +111,7 @@ int main(int argc, char** argv)
         {
             // Rationale:
             // http://stackoverflow.com/questions/25587403/boost-asio-ssl-async-shutdown-always-finishes-with-an-error
-            ec.assign(0, ec.category());
+            ec = {};
         }
         if(ec)
             throw beast::system_error{ec};