Canvas should extend TileLayer

As per [document](http://leafletjs.com/reference.html#tilelayer-canvas), the `Canvas` interface should extend `TileLayer`

leaflet/leaflet.d.ts

@@ -3886,7 +3886,7 @@ declare module L {
             setParams(params: WMS, noRedraw?: boolean): WMS;
         }
 
-        export interface Canvas {
+        export interface Canvas extends TileLayer {
             /**
               * You need to define this method after creating the instance to draw tiles;
               * canvas is the actual canvas tile on which you can draw, tilePoint represents