def _build_fields(self, defaults, fields):
        """Return provided fields including any in defaults

        >>> f = LogstashFormatter()
        # Verify that ``fields`` is used
        >>> f._build_fields({}, {'foo': 'one'}) == \
                {'foo': 'one'}
        True
        # Verify that ``@fields`` in ``defaults`` is used
        >>> f._build_fields({'@fields': {'bar': 'two'}}, {'foo': 'one'}) == \
                {'foo': 'one', 'bar': 'two'}
        True
        # Verify that ``fields`` takes precedence
        >>> f._build_fields({'@fields': {'foo': 'two'}}, {'foo': 'one'}) == \
                {'foo': 'one'}
        True
        """
        return dict(list(defaults.get('@fields', {}).items()) + list(fields.items()))