Fix escaping of interpolated attribute values

Attribute values with interpolated strings are not totally escaped.

```jade
a(href="/?t=1&u=1&v=1") foo
- auto params = "t=1&u=1";
a(href="/?#{params}&v=1") foo
```

```html
<a href="/?t=1&amp;u=1&amp;v=1">foo</a>
<a href="/?t=1&amp;u=1&v=1">foo</a>
```