Update IntIntMap.java

Method get is broken.
Ex : Using get(0, -1) on a new IntIntMap would return 0 (zeroValue) instead of that -1 (defaultValue) because there isn't the hasZeroValue check like in the remove method.

There is also a chance that other methods like getStash or getAndIncrement are affected too.
Merge pull request #376 from Ricorei/patch-1

Fix for get (int key, int defaultValue) method in IntIntMap.java