x_value_formatter property is not being used by render_table().

The function in `x_value_formatter` property is not being used by `render_table` while generating the `thead`, instead lxml throws `ValueError` exception as its expecting `str`

Start by tracing render_table() where it generates the thead and inspect how x_value_formatter is handled before the lxml call. Reproduce the ValueError with a formatter, then verify that the formatter is used for the header values and that table rendering completes successfully.

Written by the indexing model from the issue text.