Describe the bug
jq-1.6 behaves differently (and as I read the manual, incorrectly) when running map_values with a filter which includes an alternate //.

To Reproduce

% jq-1.5 'map_values( tonumber? // tostring)' <<< '{"fish": "food", "account": "1234"}'
{
  "fish": "food",
  "account": 1234
}
% jq-1.6 'map_values( tonumber? // tostring)' <<< '{"fish": "food", "account": "1234"}'
{
  "fish": "food"
}

Expected behavior
Expected jq-1.5 output to appear in both cases

Environment (please complete the following information):

• MacOS Mojave
• jq version 1.5.3 and 1.6 (poured from homebrew bottles)