title: Binding angular formly select option not work
body: <p>I'm trying to bind formly select type options using this code :</p>
  <pre><code>      fieldGroup: [
      {
        key: 'TimeOffTypeID',
        type: 'select',
        className: 'flex-40 padding-10',
        templateOptions: {
          label: 'نوع مرخصی',
          placeholder: 'نوع مرخصی',
          required: true,
          options: this.getTimeoffType,
          valueProp: 'TimeOffTypeID',
          labelProp: 'TimeOffTypeName',
        },
  </code></pre>
  <p>and</p>
  <pre><code>    types$: BehaviorSubject&lt;any[]&gt; = new BehaviorSubject&lt;any[]&gt;([]);
       
      public get getTimeoffType(): Observable&lt;any[]&gt; {
  
      return this.types$.asObservable();
      }
  </code></pre>
  <p>and data Serrvice</p>
  <pre><code>      getTimeoffTypes() {
  
  this.base
    .get(
      Controller.TIMEOFFTYPE,
      {
        TimeOffTypeID: 0,
      },
      Actions.SEARCH
    )
    .subscribe(({ result }) =&gt; {
      console.log(result)
      this.types$.next(result);
      
    })
  </code></pre>
  <p>}</p>
  <p>The result has my data but this data is not bind to form select options</p>