fix(runtime-core): assign value after children are render

[pikax]

fix #1318

Problem is caused when the select.value is set, but there's no children options, so the value cannot be set. After the first child is render the select will reset to it, but if you bind value to other value, it will ignore.

The implementation is in runtime-core but the test is on runtime-dom because is a bit trickier to test with test-utils

[yyx990803]

We shouldn't have DOM-specific special cases in runtime-core. Actually, we just need to mount children before setting the props (which is Vue 2's behavior)

[pikax]

That's definitely much better 👍