Allow named animation speed to be 0. Fixes #6579.

Author: jryans

src/effects.js

@@ -254,7 +254,7 @@ jQuery.extend({
 		};
 
 		opt.duration = jQuery.fx.off ? 0 : typeof opt.duration === "number" ? opt.duration :
-			jQuery.fx.speeds[opt.duration] || jQuery.fx.speeds._default;
+			opt.duration in jQuery.fx.speeds ? jQuery.fx.speeds[opt.duration] : jQuery.fx.speeds._default;
 
 		// Queueing
 		opt.old = opt.complete;