Content of Minecart with Spawner is not sent to the client

The bug

The mob inside a minecart with spawner is never rendered, even if the spawner is customized.

How to reproduce

1. Summon a minecart with spawner that spawns mobs other than pigs
2. Notice the lack of a miniature

(A spawner minecart that is not customised will spawn pigs, but requires light and grass to actually spawn them. They too lack a mob inside).

Note

Note that since 15w33a, no miniature is rendered at all; prior to this verison, a pig was rendered. The attached screenshot is from before 15w33a.

Code analysis

Code analysis by Marcono1234 can be found in this comment.