auto_width sets 0 widths for non-string type column headers

[foster999]

Describe the bug
Using datetime type headers in the table. columns leads to column widths being set to 0.

Expected behavior
Column widths are left as default if they're assigned to 0 and/or a warning is raised to tell the user that they should parse the header objects to strings.

Should also clarify in docs that only strings/rich-text should be used in all non-data elements.

[foster999]

The default constructor {'strings_to_numbers': True} means that even strings in index columns might also be converted to numbers by XlsxWriter. As such, auto_width needs to take into account number of digits in numbers. Alternatively, we could try switching this to False and making it clear that users should pass strings to these columns.

Above issue still stands for heading types - warning might be best