feat bulk operation: don't crash on pristine document

[H--o-l]

Database.update_docs(ids=[...]) operation was working without crash only if all documents were either modified or created. But if only a subset of documents are really modified inside the bulk operation it crashed with the error:

File "/var/coatl/lib64/python3.8/site-packages/aiocouch/bulk.py", line 59, in __aexit__
    assert status["id"] == doc.id
AssertionError

I think it's cool to improve it because: If you have foo and baz documents in your db, and want to update both, but foo already have the update. It's a nice shortcut to update both without having to exclude foo from the bulk operation (see unit test for an example).

[codecov[bot]]

Codecov Report

Merging #32 (240eb62) into master (510260c) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   95.93%   95.93%           
=======================================
  Files          11       11           
  Lines         762      763    +1     
=======================================
+ Hits          731      732    +1     
  Misses         31       31           

Impacted Files	Coverage Δ
aiocouch/bulk.py	100.00% <100.00%> (ø)

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[H--o-l]

Thanks @bmario for the quick review and merge! Is it possible for you to upload a new release with this change in a "not to far" future?

[bmario]

Done.

[H--o-l]

Thanks!