Add a passing test to try to repro #1513

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/databind/ser/std/MapSerializer.java

@@ -950,7 +950,8 @@ protected Map<?,?> _orderEntries(Map<?,?> input, JsonGenerator gen,
         if (input instanceof SortedMap<?,?>) {
             return input;
         }
-        // [databind#1411]: TreeMap does not like null key...
+        // [databind#1411]: TreeMap does not like null key... (although note that
+        //   check above should prevent this code from being called in that case)
         if (input.containsKey(null)) {
             TreeMap<Object,Object> result = new TreeMap<Object,Object>();
             for (Map.Entry<?,?> entry : input.entrySet()) {

src/test/java/com/fasterxml/jackson/databind/ser/TestMapSerialization.java

@@ -3,6 +3,7 @@
 import java.io.*;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentSkipListMap;
 
 import com.fasterxml.jackson.annotation.*;
 import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
@@ -284,4 +285,16 @@ public void testNullJsonInTypedMap691() throws Exception {
         String json = mapper.writeValueAsString(map);
         assertEquals("{\"@class\":\"java.util.HashMap\",\"NULL\":null}", json);
     }
+
+    // [databind#1513]
+    public void testConcurrentSkipListMap() throws Exception
+    {
+        Map<String,String> input = new ConcurrentSkipListMap<String,String>();
+        input.put("a", "b");
+        input.put("x", "y");
+        String json = MAPPER
+                .writer().with(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS)
+                .writeValueAsString(input);
+        assertEquals(aposToQuotes("{'a':'b','x':'y'}"), json);
+    }
 }