Withers can be trapped inside of bedrock

Withers can be trapped inside of bedrock if they are summoned directly below the bedrock, leading to them being easily farmed using the nether roof or end portal.

Steps to reproduce

• Create a bedrock layer

  /setblock ~ ~-1 ~ bedrock
  /fill ~-2 ~2 ~-2 ~2 ~2 ~2 bedrock

• Summon a wither in the same spot

  /summon wither

• The wither will be unable to escape