Fix #411, change io.save to io.replace_one

[davidlatwe]

Pymongo collections.save() is deprecated since version 3.0. According to the document, we should change to use insert_one or replace_one.

• Changed to use replace_one since we use io.save to overwrite full document.
• Add a deprecation note in io.save doc string.

411

[mottosso]

Thanks, I was also eyeing this one but wasn't sure what to replace it with. This looks good to me. 👍

[davidlatwe]

Oh, wait. I forgot the case that we create new document. Will patch this later.

[davidlatwe]

Ah, no. We are safe to merge this :) The time we call replace_one, documents were already exists in those cases. And I just tested it :P

[davidlatwe]

Will merge this tomorrow if no objections :)