Move this._layer initialization to _initialDraw

This fixes a bug where, when the location filter was added,
removed, and re-added to a map, it would not display. This
happened because the re-add would reinitialize this._layer
but not call _initialDraw, which sets up this._layer.

src/locationfilter.js

@@ -114,7 +114,6 @@ L.LocationFilter = L.Class.extend({
 
     onAdd: function(map) {
         this._map = map;
-        this._layer = new L.LayerGroup();
 
         if (this.options.enableButton || this.options.adjustButton) {
             this._initializeButtonContainer();
@@ -285,6 +284,8 @@ L.LocationFilter = L.Class.extend({
             return;
         }
 
+        this._layer = new L.LayerGroup();
+
         // Calculate filter bounds
         this._calculateBounds();