Failing test: withComponent(...).attrs is not a function
Failing test: withComponent(...).attrs is not a function

Should it be possible to add `attrs` after overriding `withComponent`? I
found a test for `withComponent` followed by `extend`, but none for
`attrs`, so I added one:

```js
const Parent = styled.button`
  color: red;
`
const Child = Parent.withComponent('a').attrs({
  href: '/test'
})``
```

Currently this throws `TypeError: Parent.withComponent(...).attrs is not
a function`.

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