fix(keep-alive): bug prune oldest entry

[jasmineMu]

prune oldest entry

The same component(keep-alive) is created many times when max is set
（keepalive render方法中，this._vnode仍为上一次的组件，并不是当前的）

detailed description of the bug
Live demo :

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[posva]

Is this #9972 what you are trying to fix? If so, could you add a test?

[posva]

Thanks! I won't be able to give it a try yet but maybe another member has the time before me!