fix(VTabs): correctly set active state with exact prop

[Florent-Bouisset]

The v-tabs component didn't move the slider if the v-tab had the property "exact"

Description

After investigating, it came from the mixin routable,
The value of "exactActiveClass" was not checked before assigning to the path, resulting in

'_vnode.data.class.undefined
I change the code to fall back on proxyClass if exactActiveClass is undefined
As a precaution I did the same for activeClass even if it's not needed for resolving this issue

the bug was introduced by PR #14207

Motivation and Context

Resolves #14431
Resolves #14459

How Has This Been Tested?

didn't add unit tests

Markup:

Playground.vue

<template>
  <v-container>
    <v-tabs>
      <v-tab
        to="/page1"
      >page1</v-tab>
      <v-tab
        to="/page2"
      >page2</v-tab>
    </v-tabs>
    <v-btn to="/page2">Go to page 2</v-btn>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
    }),
  }
</script>

Types of changes

• [x ] 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:

• [ x] The PR title is no longer than 64 characters.
• [ x] 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).
• [ x] My code follows the code style of this project.
• [ x] I've added relevant changes to the documentation (applies to new features and breaking changes in core library)