title: AngularFire2 firestore take(1) on doc valueChanges
body: <p>Using AngularFire2 I try to get data from a single Firestore document. <strong>What works is</strong>:</p>
  
  <pre><code>this.addressRef = afs.doc&lt;Address&gt;('addresses/key')
  this.addressRef.valueChanges().subscribe(val =&gt; {......}
  </code></pre>
  
  <p>I want to run the subscription code just once. Normally I use take(1) to achieve this (works with the Firebase DB). <strong>But this code</strong>:</p>
  
  <pre><code>this.addressRef.valueChanges().take(1).subscribe(val =&gt; {......}
  </code></pre>
  
  <p><strong>gives error</strong>: TypeError: this.addressRef.valueChanges(...).take is not a function.</p>
  
  <p>VS Code is listing the take action. Is this a bug in AngularFire2, am I doing this wrong or is there an other (better) way to stop the subscription after the data is fetched? I also tried topromise but this also failed.</p>