Code button in the demo not working properly

Hi Guardian team!

Using Chrome 51 on Windows.

When I try your current demo, the Code button is not working correctly. It is like the editor did't recognize that some text was already wrapped in <code>, so it keeps wrapping it over and over instead of removing the code formatting after second click.

How to reproduce:

1. Open the demo
2. Select the word World
3. Press Code button
4. Press Code button again

Expected: The selected word would be wrapped in <code> and then unwrapped. Actual: The selected word is wrapped twice (<code><code>World</code></code>)