fix default attribute elt determination algorithm

This PR fixes the algorithm that is used to determine if the default attribute element must be used (i.e. created) when passing content to a component.

As an example:
``` 
// in this case the default <@body> element will be automatically created
<#panel head="Panel B ({m.text}!)">
    {m.text}! <a onclick="{update(1)}">Update model</a> 
</#panel>

// here it is present, so no need to create it
<#panel head="Panel B ({m.text}!)">
    <@body>
        {m.text}! <a onclick="{update(1)}">Update model</a> 
    </@body>
</#panel>

// and the previous algorithm had a bug in this case
<#panel head="Panel B ({m.text}!)">
    {m.text}! <hr/>  <a onclick="{update(1)}">Update model</a> 
</#panel>
```