Merge strategy using conditional blocks around head tags

There seems to be an issue using conditional tags in the layout template:
```html
<head>
    <th:block th:if="${false}">
        <title>test</title>
    </th:block>
</head>
```
The whole condition will be ignored and the title inside will be processed/rendered.
However this is working:
```html
<head>
    <title th:if="${false}">test</title>
</head>
```

Tested with version 2.5.1.