PDFBOX-5660: implement toString for better debugging

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1930839 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

xmpbox/src/main/java/org/apache/xmpbox/type/TypeMapping.java

@@ -416,6 +416,12 @@ public Cardinality card()
             {
                 return card;
             }
+
+            @Override
+            public String toString()
+            {
+                return "{type: " + type + ", card: " + card + '}';
+            }
         };
     }
 }