fix w3m image test.

lib/widgets/image.js

@@ -50,6 +50,8 @@ function Image(options) {
     OverlayImage.call(this, options);
     return this;
   }
+
+  throw new Error('`itype` must either be `ansi` or `overlay`.');
 }
 
 Image.prototype.__proto__ = Box.prototype;

test/widget-image.js

@@ -16,7 +16,7 @@ var file = process.argv[2] || __dirname + '/test-image.png';
 
 var image = blessed.image({
   parent: screen,
-  itype: 'w3m',
+  itype: 'overlay',
   left: 'center',
   top: 'center',
   width: 'shrink',