Force the value of a datacite3 key to be a list.

    >>> _pluralize(xml_input['authors'], 'author')
    ['Sick, Jonathan', 'Economou, Frossie']

    Background
    ----------
    When `xmltodict` proceses metadata, it turns XML tags into new key-value
    pairs whenever possible, even if the value should semantically be treated
    as a `list`.

    For example

    .. code-block:: xml

       <authors>
         <author>Sick, Jonathan</author>
       </authors

    Would be rendered by `xmltodict` as::

       {'authors': {'author': 'Sick, Jonathan'}}

    While

    .. code-block:: xml

       <authors>
         <author>Sick, Jonathan</author>
         <author>Economou, Frossie</author>
       </authors

    is rendered by `xmltodict` as::

       {'authors': [{'author': ['Sick, Jonathan', 'Economou, Frossie']}}

    This function ensures that values are *always* lists so that they can be
    treated uniformly.

    Parameters
    ----------
    value : obj
        The value of a key from datacite metadata extracted by `xmltodict`.
        For example, `xmldict['authors']`.
    item_key : str
        Name of the tag for each item; for example, with the `'authors'` key
        the item key is `'author'`.

    Returns
    -------
    item_values : list
        List of values of all items.

def _pluralize(value, item_key):
    """"Force the value of a datacite3 key to be a list.

    >>> _pluralize(xml_input['authors'], 'author')
    ['Sick, Jonathan', 'Economou, Frossie']

    Background
    ----------
    When `xmltodict` proceses metadata, it turns XML tags into new key-value
    pairs whenever possible, even if the value should semantically be treated
    as a `list`.

    For example

    .. code-block:: xml

       <authors>
         <author>Sick, Jonathan</author>
       </authors

    Would be rendered by `xmltodict` as::

       {'authors': {'author': 'Sick, Jonathan'}}

    While

    .. code-block:: xml

       <authors>
         <author>Sick, Jonathan</author>
         <author>Economou, Frossie</author>
       </authors

    is rendered by `xmltodict` as::

       {'authors': [{'author': ['Sick, Jonathan', 'Economou, Frossie']}}

    This function ensures that values are *always* lists so that they can be
    treated uniformly.

    Parameters
    ----------
    value : obj
        The value of a key from datacite metadata extracted by `xmltodict`.
        For example, `xmldict['authors']`.
    item_key : str
        Name of the tag for each item; for example, with the `'authors'` key
        the item key is `'author'`.

    Returns
    -------
    item_values : list
        List of values of all items.
    """
    v = value[item_key]
    if not isinstance(v, list):
        # Force a singular value to be a list
        return [v]
    else:
        return v