Add a test to show that #2306 was already implemented

src/test/java/com/fasterxml/jackson/databind/ser/jdk/MapKeySerialization2306Test.java

@@ -0,0 +1,35 @@
+package com.fasterxml.jackson.databind.ser.jdk;
+
+import java.util.Collections;
+import java.util.Map;
+
+import com.fasterxml.jackson.annotation.JsonValue;
+import com.fasterxml.jackson.databind.*;
+
+public class MapKeySerialization2306Test extends BaseMapTest
+{
+    static class JsonValue2306Key {
+        @JsonValue
+        private String id;
+
+        public JsonValue2306Key(String id) {
+            this.id = id;
+        }
+    }
+
+    /*
+    /**********************************************************************
+    /* Test methods
+    /**********************************************************************
+     */
+
+    private final ObjectMapper MAPPER = newJsonMapper();
+
+    public void testMapKeyWithJsonValue() throws Exception
+    {
+        final Map<JsonValue2306Key, String> map = Collections.singletonMap(
+                new JsonValue2306Key("myId"), "value");
+        assertEquals(aposToQuotes("{'myId':'value'}"),
+                MAPPER.writeValueAsString(map));
+    }
+}