Fix required argument validation

[jefersondaniel]

When a required argument is followed by an optional one, then the required argument is not validated. Example:

from cleo import Command


class HelloWorldCommand(Command):
    """
    Hello World Command

    app:hello_world
        {required_arg : Some required argument}
        {non_required_arg? : Some non required argument}
        {--option=5 : Some option that defaults to 5}
    """

    def handle(self):
        self.line('Hello world')