Support auto-prefixed style value as array (client/ssr)

[fnlctrl]

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:

When exploring css-in-js solutions for vue, I came across the fact that some auto prefixed style objects (for example, by postcss-js) cannot be directly used as style option on elements, when they have an array as value, e.g.

{ display: ['-webkit-box', '-webkit-flex', '-ms-flexbox', 'flex'] }

...which happens when a rule value needs to be prefixed, other examples:

{ backgroundImage: ['radial-gradient(...)', '-webkit-radial-gradient(...)']
{ cursor: ['-webkit-grab','grab'] }

This PR adds support for directly consuming such objects as style option.