autosave creates new articles repeatedly

[yedpodtrzitko]

steps to reproduce

• open form to create a new article
• fill all required fields
• wait for autosave (article is created)
• wait for another autosave

  current behaviour

• another article is created

  expected behaviour

• article created on 1st autosave is updated

[yedpodtrzitko]

update: There was another bug in ella-hub, which made possible to save more identical articles (uniqueness is checked in model's clean(), which was not called). Now clean() is called manually, so duplicate articles are not created anymore - it returns response w/ code 400. But still - there should be performed PATCH request instead of POST to update the original article.