Rewrite json_object_object_add to replace just the value if the key already exists so keys remain valid.

This is particularly useful when replacing values in a loop, since it allows
 the key used by json_object_object_foreach to continue to be used.

tests/testReplaceExisting.expected

@@ -0,0 +1,9 @@
+Key at index 0 is [foo1]
+Key at index 1 is [foo2]
+replacing value for key [foo2]
+Key at index 2 is [foo3]
+==== second loop starting ====
+Key at index 0 is [foo1]
+Key at index 1 is [foo2]
+pointer for key [foo2] does match
+Key at index 2 is [foo3]