Comma separator may not be encoded

[Gnuk]

I see some issues with arrayFormat: 'comma':

What happens today:

qs.stringify({list: ['foo,bar', 'hello', 'world']}, { arrayFormat: 'comma' });

returns:

list=foo%2Cbar%2Chello%2Cworld

instead of:

list=foo%2Cbar,hello,world

Suggested test case (i did it in jest/jasmine format):

it('Should put comma when arrayFormat is comma', () => {
  expect(qs.stringify({list: ['foo,bar', 'hello', 'world']}, { arrayFormat: 'comma' })).toBe('list=foo%2Cbar,hello,world');
});

[ljharb]

Seems related to #237.

[Ugzuzg]

The solution would be to apply the encoder to each value of the array before joining them with a comma. I haven't found an easy way to do that without making major changes to the stringify function.

[ljharb]

Although I prefer to avoid it, major changes to the implementation are reasonable if all the current tests pass.