Form submission should save number fields as numbers in JSON

In order for this event handler to work, `cost`'s type must be an integer.

``` python
if data.cost >= 500:
change_state('Awaiting CFO Approval')
```

Which means the form data grabber should recongise the `` and save it as a number. This is like the one time when JavaScript is easier :P I assume there's no way to enable loose typing in event handlers, @mjumbewu, but worth asking..

首先定位表单数据获取器以及对 的处理。确认提交的数值在 JSON 中以数字形式存储，并确认所示的 Python 比较与 data.cost 能按预期工作。

由索引模型根据 Issue 内容生成。