Type (tag name) selector doesn't recognize type with uppercase letter

For example the following code:
```html
<!DOCTYPE HTML>
<style>
  DIV {
    display: inline-block;
    background: green;
    width: 100px;
    height: 100px;
  }
</style>
<div></div>
<DIV></DIV>
```

Per spec it should show two green blocks, while Servo (or at least Stylo) shows empty.

This is probably an issue in selectors. cc @SimonSapin 