Make asset tags accept nested liquid blocks

For example, the following code will work:

    {% css_asset_tag global %}
      {% for path in page.stylesheets %}
        - {{ path }}
      {% endfor %}
    {% endcss_asset_tag %}

I didn't included tests for this, since it seemed hard to write
such tests with the project's current test patterns. I made a small
change to a spec file just to make the current tests not fail.