Update attribution when adding map if empty

This fixes the attributions not being fetched and not showing initially
when the map is added to the DOM and the layer is added to the map at a
later point (e.g. on a button click).

Author: agorf

leaflet-bing-layer.js

@@ -154,6 +154,11 @@ L.TileLayer.Bing = L.TileLayer.extend({
   onAdd: function (map) {
     map.on('moveend', this._updateAttribution, this)
     L.TileLayer.prototype.onAdd.call(this, map)
+
+    if (this._attributions.length === 0) {
+      this._updateAttribution()
+    }
+
     this._attributions.forEach(function (attribution) {
       map.attributionControl.addAttribution(attribution)
     })