Button text isn't centered vertically

This is because Servo treats `<button>` as a regular old inline-block, while Gecko, Blink, and WebKit don't. They have some sort of special handling for `<button>` height. I couldn't find this anywhere in the HTML5 spec.

This breaks ups.com.

```
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<button style="height: 50px; padding: 0 6px; background: lavender">Continue</button>
```
