Wolves with the Owner of a non-player are spawned sitting

The Bug:

Summoning a wolf with the Owner value set to either the name or UUID of a non-existent player will cause the wolf to be spawned sitting. This does not change even if the Sitting value is set to 0b.

The bug can also be reproduced by summoning a wolf with the Owner value set to the UUID of another mob in the same world.

Steps to Reproduce:

1. Summon a wolf with the following command:

   /summon wolf ~ ~ ~ {Owner:"PLAYER_NAME",Sitting:0b}

     The wolf spawns standing up.

2. Summon a wolf with the following command:

   /summon wolf ~ ~ ~ {Owner:"////",Sitting:0b}

   The wolf spawns sitting.