Fix: "duplicate-arguments-array" should not interfere with "option.nargs" - #164

Description

#closes #118

var args = parse(
    'foo -p x y', {
        narg: { p: 2 },
        configuration: {
            'duplicate-arguments-array': false,
        }
    }
);
console.log(args);         //  { _: [ 'foo' ], p: 'y' }    <== incorrect

the correct output should be:

• 'foo -p x y' => { _: [ 'foo' ], p: [ 'x', 'y' ] }
• 'foo -p x y -p a b' => { _: [ 'foo' ], p: [ 'a', 'b' ] }
• 'foo -p x y z -p a b c' => { _: [ 'foo' ], p: [ 'a', 'b', 'c' ] }

Description of change

• fix setKey(): nargs has higher priority than duplicate
• add one test