allow an extended constructor as global mixin (fix vue-class-component#83)

[ktsn]

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

• Bugfix (might be a new feature)
• 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:
This PR moves a constructor check in mergeOptions to the head of the function so that we can use a extended Vue constructor not only as mixins/extends options but also as global mixin. This fixes vuejs/vue-class-component#83