Required attribute is not added into data-validation field with maxlength set using html5 module

I create a basic input using both `maxlength` and `required` attributes. Only the `length` is in the `data-validation` field of the generated input, therefore the `required` was not validated.

source:

``` html
<input type="text" required maxlength="20">
```

output:

``` html
<input type="text" required="" maxlength="20" data-validation="length" data-validation-length="max20" class="valid">
```
