NdcPatternConverter isn't GC-free

The issue (LOG4J2-1715) sought to make the NdcPatternConverter GC-free, but it looks as though that work doesn't get used. The interface StringBuilderFormattable is never checked for when formatting in the convertor, and as a result the builder is left to fend for itself, resorting ultimately to String.valueOf.

I believe the code should end up looking something like this.

public void format(final LogEvent event, final StringBuilder toAppendTo) {
    ThreadContext.ContextStack stack = event.getContextStack();
    if (stack instanceof StringBuilderFormattable) {
        ((StringBuilderFormattable) stack).formatTo(toAppendTo);
    } else {
         toAppendTo.append(stack);
    }
}

Happy to submit a patch if this makes sense or for someone else to. Still getting my head around Log4j internals so confirmation of this would be helpful.