fix(ssr): prevent rendering duplicate css references

Created by: katarzyna-stachura-orba

There is a chance that the same css file is included in initial and async array, causing a element pointing to the same stylesheet to be included multiple times. This fix ensures that cannot happen. Since multiple build systems exist, this defensive approach avoids any issues with how files are generated regardless of which one is being used by a given project.

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

Other information: