`insertCss` constraint

Aloha everyone,

during test coverage of my component i found the problem with shallow rendering. For example code using enzyme:

const StyledComponent = withStyles('')(
  class Foo extends Component {
    render() {
      return <div />;
    }
  }
);

expect(shallow(<StyledComponent />).html()).to.equal('<div></div>');

After execute this code, test return error: Cannot read property 'apply' of undefined. I found the cause of this error, and change withStyles module for fix error while maintaining backward compatibility.

About reason of error. I think this insertCss as required property in propTypes is unjustified constraint, because when you need create isolated test of component you can’t do this.

What you think about this issue?

Thanks!

P.S. if maintainer or community decide that actually problem, i can do PR for resolve this issue.

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[QuentinDejean]

Same here, thanks guys!

[frenzzy]

Maybe withStyles higher-order component should not require context.insertCss and just warn if it was not provided, ref #63

@koistya what do you think?