Add failing test case with nil values in #sync

[ArthurN]

Here is the failing test case based on discussion at https://github.com/apotonick/reform/issues/362

I can try to take a stab at a solution sometime soon, unless you know exactly where the problem is.

Cheers.

[fran-worley]

Thanks for this Arthur. I can't promise when but we will get this sorted as it's not expected behaviour!