Dispenser doesn't equip non-armor item on armor stand

Summary:

When you dispense an non-armor item (other than armor pieces / shield) on armor stand with arms, the dispenser don't equip this item in the main hand on the armor stand.

Steps to reproduce:

1. Place an armor stand with arms.
2. Place a dispenser next to the armor stand.
3. Put any non-armor item in dispenser.
4. Activate the dispenser by redstone

Observed results:

Dispenser don't equip item in the main hand on the armor stand.

Expected results:

Dispenser must equip item in the main hand on the armor stand if :

• Arms are shown and,
• All the default item slots are occupied, and
• if the main hand slot is available.