title: Angular Attribute Binding via attr. results in null values when using @Attribute
body: <p>If I use an angular component like this in my template:</p>
  
  <pre><code>&lt;my-cmp selectable /&gt;
  </code></pre>
  
  <p>And my component asks the following in its constructor:</p>
  
  <pre><code>constructor(@Attribute("selectable") selectable: string){};
  </code></pre>
  
  <p>Then <code>selectable</code> will be an empty string, as expected, but when it is used like this:</p>
  
  <pre><code>&lt;my-cmp [attr.selectable]="true" /&gt;
  </code></pre>
  
  <p>This will place the attribute correctly to my tag in the DOM and result in the following:</p>
  
  <pre><code>&lt;my-cmp selectable="true" /&gt;
  </code></pre>
  
  <p>But in this case <code>selectable</code> will always have a <code>null</code> value, regardless if it is on <code>construct</code> or on <code>ngOnInit</code> and of its given value (e.g. <code>false</code>, "" or anything else will also result in <code>null</code>).</p>
  
  <p>The question here is: why?
  And more important: how to use boolean attributes in angular the best way?</p>
  
  <p>Note: I explicitly don't want to use <strong>property-binding</strong> in this case, the given "input" should be static.</p>