fix(VDataTable): divider was not showing up

[nekosaur]

Description

after #8297 old divider method of adding class to colspan was no longer working.

Motivation and Context

closes #8514, closes #7411

How Has This Been Tested?

playground

Markup:

<template>
  <div>
    <v-data-table
      :headers="headers"
      :items="desserts"
      item-key="name"
      group-by="category"
      class="elevation-1"
      show-group-by
      fixed-header
      height="400px"
    ></v-data-table>
    <v-switch v-model="$vuetify.theme.dark" label="dark"></v-switch>
  </div>
</template>

<script>
export default {
  data () {
    return {
      headers: [
        { text: 'Id', value: 'id', divider: true },
        {
          text: 'Dessert (100g serving)',
          align: 'left',
          value: 'name',
        },
        { text: 'Category', value: 'category' },
      ],
      desserts: [
        {
          id: 0,
          name: 'Frozen Yogurt',
          category: 'Ice cream',
        },
        {
          id: 1,
          name: 'Ice cream sandwich',
          category: 'Ice cream',
        },
        {
          id: 2,
          name: 'Eclair',
          category: 'Cookie',
        },
        {
          id: 3,
          name: 'Cupcake',
          category: 'Pastry',
        },
        {
          id: 4,
          name: 'Gingerbread',
          category: 'Cookie',
        },
        {
          id: 5,
          name: 'Jelly bean',
          category: 'Candy',
        },
        {
          id: 6,
          name: 'Lollipop',
          category: 'Candy',
        },
        {
          id: 7,
          name: 'Honeycomb',
          category: 'Toffee',
        },
        {
          id: 8,
          name: 'Donut',
          category: 'Pastry',
        },
        {
          id: 9,
          name: 'KitKat',
          category: 'Candy',
        },
      ],
    }
  },
}
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)