BUG GridFieldFilterHeader works without non-filterable cols

Previously relied on the presence of a last column which wasn't filterable,
commonly a GridFieldEditButton. If this wasn't present, the filter buttons
were never added, leading to the GridField JS reload request being sent
without the required button form action, so GridFieldFilterHeader->handleAction()
was never called.
Merge pull request #1982 from chillu/pulls/gridfield-filter-cols

BUG GridFieldFilterHeader works without non-filterable cols