Add toString() for ObjectIdGenerator.IdKey
diff --git a/src/main/java/com/fasterxml/jackson/annotation/ObjectIdGenerator.java b/src/main/java/com/fasterxml/jackson/annotation/ObjectIdGenerator.java
index 5897cd1..dd4936c 100644
--- a/src/main/java/com/fasterxml/jackson/annotation/ObjectIdGenerator.java
+++ b/src/main/java/com/fasterxml/jackson/annotation/ObjectIdGenerator.java
@@ -177,5 +177,10 @@
             IdKey other = (IdKey) o;
             return (other.key.equals(key)) && (other.type == type) && (other.scope == scope);
         }
+
+        @Override
+        public String toString() {
+            return String.format("[ObjectId: key=%s, type=%s, scope=%s]", key, type, scope);
+        }
     }
 }