Corrected assignment order

Method should be
````csharp
var local = _instance;
if (local == null)
{
local = new();
_instance = local;
}
return local;
````

Author: benaadams

src/Microsoft.AspNet.Http/DefaultHttpContext.cs

@@ -229,8 +229,8 @@ public override ISession Session
                 var feature = SessionFeature;
                 if (feature == null)
                 {
-                    SessionFeature = new DefaultSessionFeature();
-                    feature = SessionFeature;
+                    feature = new DefaultSessionFeature();
+                    SessionFeature = feature;
                 }
                 feature.Session = value;
             }