Fix: exception is not correctly logged when calling without message

[304NotModified]

Added unit test for bug report: #787.

Without the message argument it's calling the generic version:

error(new Exception("test")

will call:

        public void Error<T>(T value)
        {
            if (this.IsErrorEnabled)
            {
                //LogLevel level, IFormatProvider formatProvider, T value
                this.WriteToTargets(LogLevel.Error, null, value);
            }
        }

This method is interpreting the argument as message. (which is correct). I'm not sure how to fix this neat - the Exception is (again) a special case.

This is not a bug since NLog 4, but we said that since NLog 4 the exception parameter is always the first one.

N.B.: creating a special method for Exception, like Error(Exception ex) won't solve it, as this works for Exeption but not for the sub classes of Exception - those will call the generic version.

Dunno how to solve this neat. Only thing that come to mind: re-introduce the is Exception quirk which just was removed in NLog 4 (another method).

Also what should be the result of Error(ex) when the ${message} layout renderer is used?

[304NotModified]

Leave the PR like this. it fixed it and improvements can be done in future versions