Fix behaviour of arguments with default value

[gonzaponte]

This is an attempt to fix #287. Might have something to do with #247 as well.

I'm not familiar with this code, so it might break other features, although tests pass locally.

[p-ranav]

Merged in https://github.com/p-ranav/argparse/commit/d28188f4d542b07de9bad3e89aa53d8bf1a53a09. Thanks!