[C] Standardize max stack size in addItem() method - Fixes BUKKIT-4003

**The Issue:**
Currently, when using the Inventory.addItem() method, it gets the max stack size to use inconsistently, either using getMaxStackSize() from the item or getMaxItemStack() from the inventory.  

**Justification for this PR:**
Inconsistency is bad

**PR Breakdown:**
This PR standardizes the code to always use the smaller of the two. 

**JIRA Ticket:**
BUKKIT-4003- https://bukkit.atlassian.net/browse/BUKKIT-4003