Armor Stands from worlds created before 1.9 have their items shuffled and deleted when upgrading to modern versions

Armor Stands created before version 1.9 will have their items be in the wrong slots/deleted upon world upgrade.

Reproduction Steps

• Create a world in 1.8.9
• Place an armor stand
• Give it arms with /entitydata @e[r=5] {ShowArms:1}
• Give it full armor and place an item in its hands

• Use this world where steps 1-4 are already done : MC-264319.zip
• Load the world in the latest release

Observed Results

• The armor stand now holds its boots and leggins in its hands (regardless of if the armor stand had arms before)
• The item it had in hand is now in the boots slot
• The helmet and chestplate are deleted
  
  
  Expected Results
• The armor stand would be upgraded properly