Fix DEFAULT getEntitySpawnReason for /summon

[Warriorrrr]

Fixes an issue where summoned entities wouldn't have the spawn reason of COMMAND, but instead defaulted to default/natural/spawner inside the Entity#load method before the command reason could be set.

[Machine-Maker]

Does Player#respawnEnttiyOnShoulder0 suffer from the exact same problem? The nbt loading of an entity before then adding it to the world with a custom spawn reason? I think the entity's spawn reason isn't overriden with SHOULDER_ENTITY like it should.

[Warriorrrr]

The entity's original SpawnReason will be set so it won't be overridden with the SHOULDER_ENTITY one, I think keeping the original would be more in line with what the javadoc message says.