Make asset tags accept nested liquid blocks

[clee704]

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.