Remove marker.dragging when not on the map (#5295)

* Add some tests for #5293

* Fix enabling marker dragging while markers aren't on the map by removing the dragging object when not on the map.

This is a change of behavior (.dragging is only defined when a marker is on the map).

* Docs for dragging change

spec/suites/layer/marker/MarkerSpec.js

@@ -108,8 +108,18 @@ describe("Marker", function () {
 			map.removeLayer(marker);
 			// Dragging is still enabled, we should be able to disable it,
 			// even if marker is off the map.
-			marker.dragging.disable();
+			expect(marker.dragging).to.be(undefined);
+			marker.options.draggable = false;
 			map.addLayer(marker);
+
+			map.removeLayer(marker);
+
+			// We should also be able to enable dragging while off the map
+			expect(marker.dragging).to.be(undefined);
+			marker.options.draggable = true;
+
+			map.addLayer(marker);
+			expect(marker.dragging.enabled()).to.be(true);
 		});
 
 		it("changes the icon to another DivIcon", function () {

src/layer/marker/Marker.Drag.js

@@ -17,7 +17,7 @@ import {Draggable} from '../../dom/Draggable';
  * ```
  *
  * @property dragging: Handler
- * Marker dragging handler (by both mouse and touch).
+ * Marker dragging handler (by both mouse and touch). Only valid when the marker is on the map (Otherwise set [`marker.options.draggable`](#marker-draggable)).
  */
 
 export var MarkerDrag = Handler.extend({

src/layer/marker/Marker.js

@@ -96,6 +96,7 @@ export var Marker = Layer.extend({
 			this.options.draggable = true;
 			this.dragging.removeHooks();
 		}
+		delete this.dragging;
 
 		if (this._zoomAnimated) {
 			map.off('zoomanim', this._animateZoom, this);