JsonUriHandler: allow POST updates without child

JsonUriHandler: allow POST updates without child

updates to properties used to be
POST /json/some/property/path
{
  name: 'somechild',
  value: 'somevalue'
}

which required some ugly path hacking when directly updating a node.
now, this works too (and in a probably more intuitive way)
POST /json/some/property/path/somechild
{
  value: 'somevalue'
}