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

   admin/views/assets/javascripts/qor/qor-fixer.js

   Line 91 in 2c1a657

   $(this).outerWidth(headerWidth[i]);

2. thead's top is not being reset after scroll back
   find it can be solve here

   admin/views/assets/javascripts/qor/qor-fixer.js

   Line 107 in 2c1a657

   $thead.css({top: pageTop});