HTML entities in text get lost.

Hi. I have SVG files in which tspans contain text with HTML entities, for example:

```
<tspan font-size="1.431245647756">
    P&amp;I BATTERY LIMIT
</tspan>
```

Optimized SVG substitute the HTML entities with their char:

```
<tspan font-size="1.43">
    P&I BATTERY LIMIT
</tspan>
```

It seems that sax does the conversion, so the text is already broken before passing through the plugins.
