fixed_rows isn't working with fixed column widths

[chriddyp]

This might be a regression. Using Dash 1.12.0

Originally reported here: https://community.plotly.com/t/dash-1-12-datatable-format-issues-when-using-fixed-header/39120

Columns should be fixed at 180px, but instead they are responsive:

This example was taken from the latest widths chapter. cc @Marc-Andre-Rivet

[chriddyp]

Fixed in #793