Sets fields on the arg rdfvalue object.

def _SetField(self, args, type_info, value):
    """Sets fields on the arg rdfvalue object."""
    if hasattr(type_info, "enum"):
      try:
        coerced_obj = type_info.enum[value.upper()]
      except KeyError:
        # A bool is an enum but serializes to "1" / "0" which are both not in
        # enum or reverse_enum.
        coerced_obj = type_info.type.FromHumanReadable(value)
    else:
      coerced_obj = type_info.type.FromHumanReadable(value)
    args.Set(type_info.name, coerced_obj)