title: Liquid: check if object contains certain string
body: <p>In a shopify template, I create a link with</p>
  <pre><code>&lt;a href=&quot;{{ featured.url | within: collection }}&quot; class=&quot;grid-link-slider&quot;&gt;
  </code></pre>
  <p>I need to check if the url contains a certain word &quot;postcard&quot;, and if true, set a second class. Unfortunately,</p>
  <pre><code>&lt;a href=&quot;{{ featured.url | within: collection }}&quot; class=&quot;grid-link-slider {% if {{ featured.url | within: collection }} contains &quot;postcard&quot; %}grid-link-postcard{% endif %}&quot;&gt;
  </code></pre>
  <p>is not working, &quot;contains&quot; can only be used for strings and also the syntax for sure is invalid.
  Is there a way to solve this?</p>