title: async pipe into src image attribute - Angular 2+
body: <p>i have problem with observable and async pipe. I have input in my component with Observable. In controller and on subscribe into observable in ngOnInit im get valid values. Problem is in template, I do not get valid values on template. I want to have async in image src attribute.</p>
  
  <p>TvShowComponent (return Observable)</p>
  
  <pre><code>    public getCoverLink(): Observable&lt;string&gt; {
        return this.tvShowForm.get('cover').valueChanges.pipe(map((coverLink:  string) =&gt; {
          return coverLink;
        }));
    }
  </code></pre>
  
  <p>TvShowComponent template (using nested component)</p>
  
  <pre><code>&lt;fp-cover-thumbnail [coverLink$]="getCoverLink()"&gt;&lt;/fp-cover-thumbnail&gt;
  </code></pre>
  
  <p>CoverThumbnailComponent (with Input())
      @Input() public coverLink$: Observable;</p>
  
  <pre><code>ngOnInit() {
      this.coverLink$.subscribe(data =&gt; {
          console.log(data); // works here
      });
  }
  </code></pre>
  
  <p>CoverThumbnailComponent template (but not works here :( )</p>
  
  <pre><code>&lt;div class="cover-thumbnail-container"&gt;
  &lt;img #cover [src]="(coverLink$ | async)"&gt; &lt;!-- here not work --&gt;
  coverLink: {{ (coverLink$ | async) }} &lt;!-- also here not work --&gt;
  &lt;/div&gt;
  </code></pre>