publishedAt should be used to identify form drafts

[lognaturel]

https://github.com/ropensci/ruODK/issues/86 added filtering out unpublished forms from form lists. Looking at the tests here this uses version and hash to identify unpublished forms. However, those are only incidentally null in Central versions < 1.2 and will be populated in Central v1.2+.

From the docs:

• version: The version of this form as given in its XForms XML definition.
• hash: An MD5 sum automatically computed based on the XForms XML definition.

The intended way to identify unpublished form is to match on publishedAt of null (e.g. in Briefcase). We now see this lacks documentation and will make sure that is updated. publishedAt exists for all Central versions that include drafts. To support earlier versions (though we don't recommend it), you could first check for the existence of publishedAt. If it doesn't exist, the form can be included because there can be no drafts to filter out.

[florianm]

Sophie had a general review comment on ODK

[florianm]

Fixed by adding published_at to form_list and form_detail, and clarifying in examples how to find form drafts. Thanks for the detailed bug report, @lognaturel!

[lognaturel]

Yaaaay, Sophie! Can't wait for your first pull request! 😊