Float parsing blows up on % symbol

One of our customers' Items query is returning a percentage that has an actual `%` symbol in the float field.  E.g.

```
<PricePercent>18.0%</PricePercent>
```

The implementation of [FLOATTYPE](https://github.com/qbwc/qbxml/blob/master/lib/qbxml/types.rb#L8) uses `Float()`, which is throwing an exception for us when trying to parse `18.0%`.  Any reason it couldn't be a `to_f` there instead?
