fix label content check when the text follows enclosed input

label content following an input (e.g. checkbox) was being ignored. e.g:

```
<!DOCTYPE html>
<html lang="en">
    <head>
        <title>Test checkbox label validation</title>
    </head>
    <body>
        <p><label><input type='checkbox' value='1'> Test Label</label></p>
    </body>
</html>
```