Fix SafeNBTCopy#hasKey semantics

Fixes hasKey to check that a key actually exists, while retaining the
option to scream at keys which are requested which haven't been marked as
needing to be copied

Spigot-Server-Patches/Fix-Non-Full-Status-Chunk-NBT-Memory-Leak.patch

@@ -61,8 +61,10 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +        @Override
 +        public boolean hasKey(String s) {
-+            if (this.keys.contains(s)) {
++            if (super.hasKey(s)) {
 +                return true;
++            } else if (keys.contains(s)) {
++                return false;
 +            }
 +            throw new IllegalStateException("Missing Key " + s + " in SafeNBTCopy");
 +        }