title: How do I set default value of md-select?
body: <p>I'm using @angular/material@latest ,
  but the doc didn't say how can I set the default option of md-select.
  I tried selected attribution but didn't work;</p>
  
  <pre><code>&lt;md-select&gt;
  &lt;md-option selected value="a"&gt;a&lt;/md-option&gt;
  &lt;md-option selected value="b"&gt;b&lt;/md-option&gt;
  &lt;md-select&gt;
  </code></pre>