Creating a folder on the live 0.15.5 server did nothing visible, with no error
and nothing after a reload. The folder was real all along: FileNode/query masks
its results with document_ids(false) — resources that are *not* containers — so
it returns files and never folders, and says nothing about the omission.
FileNode/get carries no such mask, so on those servers the whole tree comes
from a single get with ids:null instead. That also stops ensureFolder making a
fresh "ihasmail" folder on every signature save, having never been able to find
the one already there.