Improved deprecation warning for change in form boolean values.

    The result is passed through 'mark_safe'.

    """

    if [v for v in attrs.values() if v is True or v is False]:

    for attr_name, value in attrs.items():

        if type(value) is bool:

            warnings.warn(

            'The meaning of boolean values for widget attributes will change in Django 1.8',

                "In Django 1.8, widget attribute %(attr_name)s=%(bool_value)s "

                "will %(action)s. To preserve current behavior, use the "

                "string '%(bool_value)s' instead of the boolean value." % {

                    'attr_name': attr_name,

                    'action': "be rendered as '%s'" % attr_name if value else "not be rendered",

                    'bool_value': value,

                },

                DeprecationWarning

            )

    return format_html_join('', ' {0}="{1}"', sorted(attrs.items()))