Don't pass innerRef prop

[frenic]

As ref is omitted by React, the same thing should happen with innerRef. This example would cause "invalid prop"-warning since invalid props are not omitted when passing a component as argument to styled.

class Shared extends React.Component {
  render() {
    return <div {...this.props} />
  }
}

const Foo = styled(Shared, {
  color: 'pink',
});

<Foo innerRef={shared => shared} />

This is not an issue when the argument to styled is a string, but this PR makes sure innerRef is omitted no matter what.

[rtsao]

Ah, nice catch. Thanks!