Issue Overview

Use of loop and autostart in causes unknown error

Steps to Reproduce (for bugs)

1. Add audio block
2. Use edit as HTML to add the loop and/or autoplay attributes to the tag.
3. Update the post.
4. message: block has been changed
5. Click edit as HTML in the block.
6. block shows unknown error. No HTML is shown
7. Switch to code editor: the string ="" is added to the controls, loop and autoplay attributes

Firefox 59.0.3, Chrome 66, Gutenberg 2.8

Expected Behavior

Preferably the addition of loop and autoplay options(checkboxes), as are available when using the audio shortcode.
At least allow for the error free addition of attributes.