Fix simplexml_mode for add op

Author: Mike McCabe

simplexml_tests.json

@@ -18,7 +18,7 @@
        "doc": { },
        "patch": [ { "op":"add", "path":"/foo/0", "value":1 } ],
        "expected": { "foo":1 },
-       "disabled": true
+       "disabled": false
      },
 
     { "comment": "simple 0-index of scalar ok",