When middle clicking a mob in creative, instead of giving you its spawn egg, it will give you a blank spawn egg. It will also switch to an existing spawn egg if you already had one in your inventory, even if its not the spawn egg to that mob.
Environment
Operating System: Linux (amd64) version 3.19.0-21-generic
Java Version: 1.7.0_79, Oracle Corporation
Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 412753696 bytes (393 MB) / 541978624 bytes (516 MB) up to 1060372480
Can confirm for 15w33b.