Wither spawning sound is not played when using wither spawn egg

The bug

When the wither spawns normally, a spawning sound (subtitle "Wither released") is played to indicate that the wither has spawned. However, a wither spawned via use of the new wither spawn egg does not play this sound.

How to reproduce

1. Create a wither via building it manually
2. Listen for a bell-like sound when it reaches full health
3. Get rid of this wither
4. Obtain a wither spawn egg
5. Use this to spawn another wither
6. Listen out for the sound mentioned previously

Expected results

The "Wither released" sound would play when the wither spawn egg is used.

Actual results

It is not.

How to fix

When a spawn egg is used, the mob's idle sound event is played. This could be fixed by making the wither spawn egg point to its spawning sound instead.