[bug] gdbm tied hashes always consider members set

[Stephane Chazelas <stephane@xxxxxxxxxxxx>]

$ zmodload zsh/db/gdbm
$ ztie -d db/gdbm -f new.db a
$ typeset -p a
typeset -A a=( )
$ echo ${+a[x]}
1
$ echo ${a[x]+yes}
yes

Members are always consider set even when they're not.

-- 
Stephane