Improve json context

- Add @EmmanuelVella fix "Fix json schema validation" into branch behat-2.X
- Add step: in JsonContext:

  Then the JSON should be invalid according to this schema:
            xxxxxxxx
   the JSON node "(?P<node>[^"]*)" should be valid according to the schema "(?P<filename>[^"]*)"
   the JSON node "(?P<node>[^"]*)" should not be valid according to the schema "(?P<filename>[^"]*)"

- Fix step `the JSON node "Item[10]" should not exist` when the node is an array.