Change the seen_non_default_actions test to using a using_default flag from _get_values.

Python used a 'argument_values is action.default' test which is prone to implementation issues (cf with pypy)

This change narrows the test (from the !=) used here and in argparse.js

Now only optional positionals qualify as 'non present'
bugs.pypy.org/issue1595
http://bugs.python.org/issue18943