Make AbstractMapCodec package-access (#1012)

Author: stIncMale

bson/src/main/org/bson/codecs/AbstractMapCodec.java

@@ -23,7 +23,7 @@
 import java.util.HashMap;
 import java.util.Map;
 
-public abstract class AbstractMapCodec<T> implements Codec<Map<String, T>> {
+abstract class AbstractMapCodec<T> implements Codec<Map<String, T>> {
 
     abstract T readValue(BsonReader reader, DecoderContext decoderContext);