is case-insensitive
        - key order is preserved
        - if initialized with a _parsed parameter, it will pull values from
          that dictionary-like object as needed (rather than making a copy).
          The _parsed dict is expected to be able to handle case-insensitive
          keys.

    If _parsed is not None, an optional _fields parameter specifies which keys
    in the _parsed dictionary are exposed.
    Nú