Expressions for bare HTML attributes break evaluation

This was reported on #pyramid by varesa. I haven't figured out why this breaks, but the tests do show the failure:

```
Expected:
    <html>
      <body>
        <input name="1" type="checkbox" checked>
        <input name="1" type="checkbox" checked="checked">
      </body>
    </html>
Got:
    <html>
      <body>
        <input name="${'foo'}" type="checkbox" checked>
        <input name="foo" type="checkbox" checked="checked">
      </body>
    </html>
```