Honour coords.z when passed to TileLayer.getTileUrl

cf #6004 #6006

Before this change, `coords.z` would not taken into account, and
was always overriden by `this._tileZoom`.
In the normal flow, `coords.z` is anyway set from `this._tileZoom`
before `getTileUrl` is called (in GridLayer._update). But this
prevents using `getTileUrl` out of this `_update` method.
There are some scenarios where this is needed (see #5822), or
(which is the case in uMap) when wanting to display a tile as
a tilelayer preview.

spec/suites/layer/tile/TileLayerSpec.js

@@ -509,4 +509,16 @@ describe('TileLayer', () => {
 			layer.setUrl(placeKitten);
 		});
 	});
+
+	describe('#getTileUrl', function () {
+		it('can call with static coords', function () {
+			// Layer is not added to the map
+			var layer = L.tileLayer('http://example.com/{z}/{x}/{y}.png');
+			var url = layer.getTileUrl({z: 1, x: 2, y: 3});
+			expect(url).to.equal('http://example.com/1/2/3.png');
+			layer.addTo(map)
+			var url = layer.getTileUrl({z: 1, x: 2, y: 3});
+			expect(url).to.equal('http://example.com/1/2/3.png');
+		});
+	});
 });

src/layer/tile/TileLayer.js

@@ -181,7 +181,7 @@ export const TileLayer = GridLayer.extend({
 			s: this._getSubdomain(coords),
 			x: coords.x,
 			y: coords.y,
-			z: this._getZoomForUrl()
+			z: this._getZoomForUrl(coords.z)
 		};
 		if (this._map && !this._map.options.crs.infinite) {
 			const invertedY = this._globalTileRange.max.y - coords.y;
@@ -210,8 +210,7 @@ export const TileLayer = GridLayer.extend({
 		e.tile.onload = null;
 	},
 
-	_getZoomForUrl() {
-		let zoom = this._tileZoom;
+	_getZoomForUrl: function (zoom) {
 		const maxZoom = this.options.maxZoom,
 		      zoomReverse = this.options.zoomReverse,
 		      zoomOffset = this.options.zoomOffset;