Cast value to string if input has maxlength option

Hi

when we do
```ruby
fill_in "test", :with => 1
```
```html
<input type="text" name="test">
```

it works fine as 1 gets casted into string. When we do the same for

```html
<input type="text" name="test" maxlength="100">
```

we get error
```
can't convert Range into Integer
```