Mixing style objects and functions messes up specificity resolution

Expected:
```tsx
const Title = styled("h1")(p => ({
background: "blue"
}));

const Title2 = styled(Title)({
background: "green"
});
```

Expected:
Title2 is green

Resulted:
Title2 is blue, because it has 2 class names and one of them loses the specificity war.

Repro:
https://codesandbox.io/s/styled-jss-playground-repro-mo3c8

Start with the linked CodeSandbox reproduction and compare the generated styles for Title and Title2. Trace how styled components combine function-based and object-based styles and how their class names affect specificity. Done means Title2 renders with a green background instead of blue, with the reproduction behavior covered by a regression test.

Written by the indexing model from the issue text.