Avoid 'separator' leaking into serializer output

Accumulator wrappers passed in array.push.bind(array) as the callback. Push
is however variadic, so the serializer info parameter passed in by the
separator handler made it into the output.

This patch avoids that by defining an explicit non-variadic callback.

Change-Id: Ie428e047645a5b8962ab51e4da618ecf9c6632da