Fixes for the LogicalThreadContext - NDC stack is wrong

This is related to issue number 455. (Link to github: https://github.com/apache/log4net/pull/12)

I was playing around with this fix recently and came across some strange behavior.
Imagine main method:

using (LogicalThreadContext.Stacks["NDC"].Push("Start"))
{
    // init all the stuff here
    _timer.TimerElapsed += TimerOnTimerElapsed;
}

An that would be it.

When I am on the timer tick event I have another context:

using (LogicalThreadContext.Stacks["NDC"].Push("timer"))
{
    Logger.Info("I am doing something here. Show me the context");
}

As a result expect to see the context of the timer like this: "timer",
instead, I see: "Start timer"

Is this correct behavior?