Describe the bug
According to README.md, we should be able to pass a className prop (I assume to the <input?) into URLInput, after an attempt to use it and not being able to find the class applied to any element, I had a look at index.js where I don't see the className prop used at all.

To Reproduce
Steps to reproduce the behavior:

1. Create a custom block type as in the example passing a custom className:

<URLInput
    className="custom-url-input"
    value={ attributes.url }
    onChange={ ( url, post ) => setAttributes( { url, text: (post && post.title) || 'Click here' } ) }
/>

Expected behavior
The className passed to be applied to one of the rendered elements.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop

• OS: macOS
• Browser: Firefox
• Version: 64

Additional context

• Default shipped with WordPress 5.0.3