fix(VDataTable): group-by should use correct slot

[nekosaur]

Description

group-by functionality skipped potential item slot

Motivation and Context

closes #10289

How Has This Been Tested?

playground, unit test

Markup:

<template>
  <v-data-table
    :headers="headers"
    :items="desserts"
    item-key="name"
    group-by="category"
    class="elevation-1"
  >
    <template v-slot:group.header="{toggle,group,isOpen}">
      <td colspan="2" class="pl-0">
        <v-btn @click="toggle()" icon>
          <v-icon>mdi-{{isOpen?'arrow-up-drop-circle-outline':'arrow-down-drop-circle-outline'}}</v-icon>
        </v-btn>
        <span class="info--text">category:</span>
        {{group}}
      </td>
    </template>
    <template v-slot:item="{item}">
      <tr @click="Alert(item.name)">
        <td>
          {{item.name}}
        </td>
        <td>
          {{item.category}}
        </td>
      </tr>
    </template>
  </v-data-table>
</template>

<script>
  export default {
    data () {
      return {
        headers: [
          {
            text: 'Dessert (100g serving)',
            align: 'left',
            value: 'name',
          },
          { text: 'Category', value: 'category' },
        ],
        desserts: [
          {
            name: 'Frozen Yogurt',
            category: 'Ice cream',
          },
          {
            name: 'Ice cream sandwich',
            category: 'Ice cream',
          },
          {
            name: 'Eclair',
            category: 'Cookie',
          },
          {
            name: 'Cupcake',
            category: 'Pastry',
          },
          {
            name: 'Gingerbread',
            category: 'Cookie',
          },
          {
            name: 'Jelly bean',
            category: 'Candy',
          },
          {
            name: 'Lollipop',
            category: 'Candy',
          },
          {
            name: 'Honeycomb',
            category: 'Toffee',
          },
          {
            name: 'Donut',
            category: 'Pastry',
          },
          {
            name: 'KitKat',
            category: 'Candy',
          },
        ],
      }
    },
    methods: {
      Alert (x) {
        alert(x)
      },
    },
  }
</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 backwards compatible changes and next for non-backwards compatible 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)