Fix json output for -name option

Fix json output for -name option

When listing just the set names,json output generated key:value pairs
in an array and not a map, which is invalid in json. Instead of

[
"name" : "test"
"name" : "test2"
]

generate

[
  { "name" : "test" },
  { "name" : "test2" }
]

Fixes bugzilla #1726.

Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>