convert.to_json on scalar should product array

[ebhills]

Currently, in order to convert a str column to JSON, we have to use the to_list wrangle. That is because to_json outputs the scalar in quotes, but not structured as an array. Let's change the behavior of to_json to wrap the scalr in an array (list) because that's the structure all other wrangles expect.

  - convert.to_json:
      input: Test # abcd

output should be ["abcd"] - currently it is "abcd"