Fixes header table issue

[annewanghy]

find 2 bugs when table is fixes header qor-table-fixed-header

1. thead th is not width enough, didn't match with body td find it can be solved here by tweak from width to min-width https://github.com/qor/admin/blob/2c1a65734f671cab0c489a7c275ae534e01c3cfa/views/assets/javascripts/qor/qor-fixer.js#L91
2. thead's top is not being reset after scroll back find it can be solve here https://github.com/qor/admin/blob/2c1a65734f671cab0c489a7c275ae534e01c3cfa/views/assets/javascripts/qor/qor-fixer.js#L107