title: agm-markers not rendering when IconUrl given
body: <p>I a trying to render icons for agm-marks using iconUrl, but nothing is showing, also i would like to the the same for Symbol syntax.</p>
  
  <p>I have looked at all the solution involving icon objects, but it is not working.</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;agm-marker-cluster [imagePath]="imagePath" [minimumClusterSize]="15"&gt;    
      &lt;agm-marker [iconUrl]="'https://github.com/Concept211/Google-Maps-Markers/blob/master/images/marker_black.png'"
          [opacity]="'1'"
          [visible]="'true'"
          *ngFor="let order of orders_data | keyvalue" 
          [latitude]="order.value.member_address_latitude"  
          [longitude]="order.value.member_address_longitude"
          [markerClickable]="true"
          (markerClick)="orderClicked(order)"
          &gt;&lt;/agm-marker&gt;
      &lt;/agm-marker-cluster&gt;
  </code></pre>