This is the fix done for issue #177.
If you're using a map<string,?>, Hive will call getMapValueElement, with a key of class Text or String, while our map only contains BinaryWritable.
I didn't check, but a map<byte,?> and a map<short,?> should have the same behavior, since we're using IntWritables instead of ShortWritables and ByteWritables.
Since we only need this iteration in these cases, I guess I can add a check before doing it.