Ray: Set direction to 0, 0, -1 by default.

src/math/Ray.js

@@ -16,7 +16,7 @@ var _normal = new Vector3();
 function Ray( origin, direction ) {
 
 	this.origin = ( origin !== undefined ) ? origin : new Vector3();
-	this.direction = ( direction !== undefined ) ? direction : new Vector3();
+	this.direction = ( direction !== undefined ) ? direction : new Vector3( 0, 0, - 1 );
 
 }