Scenario outlines make the JSON formatter fail (an exception is raised)

The scenario added in this PR with the @wip-new-core tag, does pass in v1.3.x (but it is not included in the json_formatter.feature until #700).

The problem is that in gherkin_formatter_adapter the code relies on using the gherkin_statement method on the AST classes. This does not work on the AST classes Core::Ast::ScenarioOutline nor Core::Ast::ExamplesTable.

In the case of the ScenarioOutline class, it is simple to fix, only add a default argument similar to the corresponding method in [Core::Ast::Feature](https://github.com/cucumber/cucumber-ruby-core/blob/master/lib/cucumber/core/ast/feature.rb#L24):
```
        def gherkin_statement(statement=nil)
          @gherkin_statement ||= statement
        end
```

In the case the ExamplesTable class, I see no such simple fix.