Revise default for Buffer in the browser

Back in 7d12d89a0a we added support for
Node.js global `Buffer` object, explicitly exposing it to the module
loader sandbox. The value `{}` was used in the browser, but is now
causing problems with libraries that perform feature detection.

boot/boot.js

@@ -648,7 +648,7 @@ $tw.modules.execute = function(moduleName,moduleRoot) {
 			clearInterval: clearInterval,
 			setTimeout: setTimeout,
 			clearTimeout: clearTimeout,
-			Buffer: $tw.browser ? {} : Buffer,
+			Buffer: $tw.browser ? undefined : Buffer,
 			$tw: $tw,
 			require: function(title) {
 				return $tw.modules.execute(title, name);