Fix categories with spaces as the context[@category] also needs converting

[andruby]

When using this code, the category url did not replace spaces with dashes. The added parentheses in this pull request resolved the issue.

{% for category in site.categories %}
  <a href="{% category_url category.first %}">{{ category.first }}</a>
{% endfor %}

[iamsebastian]

Do it!

[alain-andre]

@zroger any idea why this isn't merged ? I'm actually having the problem, category_url do not lowercase and replace spaces by -.