extractKey = (k) => {
  const hashIdx = "_**[]"
  if (k.indexOf(hashIdx) < 0) {
    return k
  }
  return k.split(hashIdx)[0].trim()
}
if duplicate key name existed from FormData entries,
we mutated the key name by appending a hashIdx
@param {String} k - possibly mutated key name
@return {String} - src key name