fix typo, meant to be proc returning empty hash
fix typo, meant to be proc returning empty hash

that tests passed as is (with an empty proc?) probably means nil would
work as default too? But let's leave it what we intended, which also has tests pass.

jrochkind/json_attribute,jrochkind/json_attribute,jrochkind/json_attribute,jrochkind/json_attribute