Modifying player inventories replaces armor slots with an item

[ItsSynchro]

Information

Full output of /ess version:

[11:27:46] [Server thread/INFO]: Server version: 1.15.2-R0.1-SNAPSHOT git-Tuinity-"eedc3fc" (MC: 1.15.2)
[11:27:46] [Server thread/INFO]: EssentialsX version: test-async-teleport.1
[11:27:46] [Server thread/INFO]: PlaceholderAPI version: 2.10.6
[11:27:46] [Server thread/INFO]: PermissionsEx version: 1.23.4
[11:27:46] [Server thread/INFO]: Vault version: 1.7.2-b107
[11:27:46] [Server thread/INFO]: Citizens version: 2.0.26-SNAPSHOT (build 1970)
[11:27:46] [Server thread/INFO]: Towny version: 0.96.1.10
[11:27:46] [Server thread/INFO]: EssentialsXSpawn version: test-async-teleport.1
[11:27:46] [Server thread/INFO]: You are running unsupported plugins!```

**Server log**: https://gist.github.com/ItsSynchro/a19069b102e7752fdf62556bc66158f5

**EssentialsX config**: https://gist.github.com/ItsSynchro/2dc0f860fc9b11677aae46ccc4d05054

### Details

**Description**  
When dragging out items from players inventories, armor has a chance to be replaced by the item taken out.

**Steps to reproduce**  
Open another players inventory using /invsee
Drag items out of their inventory (I specifically chose 3 stacks of dirt). Shift clicking the item won't reproduce it.
Keep attempting to take away the dirt blocks and placing them back in (this entire time don't shift click anything)

**Expected behavior**  
Not replace players armor piece with the block taken away.

**Screenshots**  
![Capture](https://user-images.githubusercontent.com/16171807/85223519-d0a00800-b391-11ea-8c51-46bba777ebd8.PNG)

[triagonal]

Near duplicate/related to #3065 - closing in favor of original issue.