title: "subscribe()" is deprecated when using "of(false)"
body: <p>I am trying to use <code>of</code> but my editor is saying that it is deprecated. How can I get this to working using <code>of</code>?</p>
  <pre><code>  public save(): Observable&lt;ISaveResult&gt; | Observable&lt;boolean&gt; {
      if (this.item) {
        return this.databaseService.save(this.userId, this.item)
      }
      return of(false)
    }
  </code></pre>
  <pre><code>  public componentSave() {
      const sub = this.userService
        .save()
        .subscribe(val =&gt; {
          // This callback shows the below error in the editor
        })
    }
  </code></pre>
  <p>When I use this, I get the following error in my editor:</p>
  <pre><code>@deprecated — Use an observer instead of a complete callback
  
  '(next: null, error: (error: any) =&gt; void, complete: () =&gt; void): Subscription' is deprecated
  Expected 2-3 arguments, but got 1.
  </code></pre>