`values_from_json` not compatible with Cromwell pair definitions

Some [old Cromwell workflows](https://github.com/g3chen/star/blob/master/test.wdl.json) define pairs as `{"Left": "value1", "Right": "value2}`. `values_from_json` claims to be Cromwell compatible but does not support this pair definition, meaning MiniWDL is unable to run these workflows out of the box.