Change IdentityManager getNextFreeId()

[SuperSimpleGuy]

Should be:

public int getNextFreeId(String key) {
    if (!identities.containsKey(key)) {
        return 0; //This shouldn't be -1, as zero is technically the next free id.
    }
    int temp = identities.get(key);
    identities.put(key, temp + 1);
    return temp;
}