fix(FitBounds): unsubscribe after unsuccessful computation

fixi an edge case problem where if user was using automatic fitBounds calculation suddenly switches to fitBounds object (e.g. LngLatBoundsLiteral) and then provided fitBounds by user would be overwritten by the active subscription.

fixes: #1810

packages/core/directives/map.ts

@@ -536,6 +536,9 @@ export class AgmMap implements OnChanges, OnInit, OnDestroy {
         }
         break;
       default:
+        if (this._fitBoundsSubscription) {
+          this._fitBoundsSubscription.unsubscribe();
+        }
         this._updateBounds(this.fitBounds, this.fitBoundsPadding);
     }
   }