title: sort by two properties
body: <p>my object looks like :</p>
  
  <pre><code>object = [[section: "section1", order: "1"],
            [section: "section2", order: "1"],
            [section: "section1", order: "2"],
            [section: "section2", order: "2"]]
  </code></pre>
  
  <p>i want to sort it to have a result like :</p>
  
  <pre><code>[[section: "section1", order: "1"],
   [section: "section1", order: "2"],
   [section: "section2", order: "1"],
   [section: "section2", order: "2"]]
  </code></pre>
  
  <p>So i need to sort by section, and in each section by order.</p>
  
  <p>That's what i'm doing :</p>
  
  <pre><code>  return Observable.from(realm
              .objects(Section.self).sorted(byProperty: "order", ascending: true)
  </code></pre>
  
  <p>The string "section.." is just for the example, it can be other thing so i can't just use the character to sort. I need a real priority on X string.</p>