Rendering a proper value for html5 attribute 'step'

This solve a issue when we have a field difined as decimal, where decimal place are greater than 4. 
For example: a field 'ratio' defined as DECIMAL(10,6)

```php
echo $this->Form->input('Unit.ratio');
``` 
now this render:
```html
<input name="data[Unit][ratio]" required="required" autocomplete="off" step="1,0E-6" type="number"  id="Unit.ratio">
```