Fix attribution text not removed when Layer is removed from map

Symptom: Switching between layers with different attributions does
not remove the attribution from the previously selected layer.

Control.Attribution keeps track of its attributions with a counter for
each text. The problem described in Leaflet#4285 is that each time the layer
is added to the map, the counter is increased by two. It's because
Layer calls Control.Attribution.addAttribution() twice, once from
addLayer() and the second time via the whenReady() callback _layerAdd().
This was not caught by the tests since the callback was never fired
(missing map.setView() call).

Fixes Leaflet#4285

spec/suites/control/Control.AttributionSpec.js

@@ -7,9 +7,17 @@ describe("Control.Attribution", function () {
 		control = new L.Control.Attribution({
 			prefix: 'prefix'
 		}).addTo(map);
+		map.setView([0, 0], 1);
 		container = control.getContainer();
 	});
 
+	function dummyLayer() {
+		var layer = new L.Layer();
+		layer.onAdd = function () { };
+		layer.onRemove = function () { };
+		return layer;
+	}
+
 	it("contains just prefix if no attributions added", function () {
 		expect(container.innerHTML).to.eql('prefix');
 	});
@@ -67,9 +75,9 @@ describe("Control.Attribution", function () {
 
 	describe('on layer add/remove', function () {
 		it('changes text', function () {
-			var fooLayer = new L.Layer();
-			var barLayer = new L.Layer();
-			var bazLayer = new L.Layer();
+			var fooLayer = dummyLayer();
+			var barLayer = dummyLayer();
+			var bazLayer = dummyLayer();
 			fooLayer.getAttribution = function () { return 'foo'; };
 			barLayer.getAttribution = function () { return 'bar'; };
 			bazLayer.getAttribution = function () { return 'baz'; };
@@ -91,9 +99,9 @@ describe("Control.Attribution", function () {
 		});
 
 		it('keeps count of duplicated attributions', function () {
-			var fooLayer = new L.Layer();
-			var fo2Layer = new L.Layer();
-			var fo3Layer = new L.Layer();
+			var fooLayer = dummyLayer();
+			var fo2Layer = dummyLayer();
+			var fo3Layer = dummyLayer();
 			fooLayer.getAttribution = function () { return 'foo'; };
 			fo2Layer.getAttribution = function () { return 'foo'; };
 			fo3Layer.getAttribution = function () { return 'foo'; };

src/layer/Layer.js

@@ -73,10 +73,6 @@ L.Map.include({
 			layer.beforeAdd(this);
 		}
 
-		if (layer.getAttribution && this.attributionControl) {
-			this.attributionControl.addAttribution(layer.getAttribution());
-		}
-
 		this.whenReady(layer._layerAdd, layer);
 
 		return this;